Chapter XV: Part 15

These values were considered to support the view that the four larger and more distant orbs shine partly by inherent lustre, and the more so as spectroscopic analysis indicates that they are each involved in a deep vapour-laden atmosphere. But certain observations furnish a contradiction to Proctor's views. The absolute extinction of the satellites, even in the most powerful telescopes, while in the shadow of Jupiter, shows that they cannot receive sufficient light from their primary to render them visible, and the darkness of the shadows of the satellites when projected on the planet's disk proves that the latter cannot be self-luminous except in an insensible degree. It is also to be remarked that, were it only moderately self-luminous, the colour of the light which it sends to us would be red, such light being at first emitted from a heated body when its temperature is raised. Possibly, however, the great red spot, when the colouring was intense in 1878 and several following years, may have represented an opening in the Jovian atmosphere, and the ruddy belts may be extensive rifts in the same envelope. If Jupiter's actual globe emitted a good deal of heat and light we should probably distinguish little of it, owing to the obscuring vapours floating above the surface. Venus reflects relatively more light than Jupiter, and there is little doubt that the albedo of a planet is dependent upon atmospheric characteristics, and is in no case a direct indication of inherent light and heat.

The colouring of the belts appears to be due to seasonal variations, for Stanley Williams has shown that their changes have a cycle of twelve years, and correspond as nearly as possible with a sidereal revolution of Jupiter. The variations are of such character that the two great equatorial belts are alternately affected; when the S. equatorial belt displays maximum redness the N. equatorial is at a minimum and vice versa.

The most plausible hypothesis with regard to the red spot is that it is of the nature of an island floating upon a liquid surface, though its great duration does not favour this idea. But it is an open question whether the belts of Jupiter indicate a liquid or gaseous condition of the visible surface. The difficulty in the way of the liquid hypothesis is the great difference in the times of rotation between the equatorial portions of the planet and the spots in temperate latitudes. The latter usually rotate in periods between 9 h. 55 m. and 9 h. 56 m., while the equatorial markings make a revolution in about five minutes less, 9 h. 50 m. to 9 h. 51 m. The difference amounts to 7.5° in a terrestrial day and proves that an equatorial spot will circulate right round the enormous sphere of Jupiter (circumference 283,000 m.) in 48 days. The motion is equivalent to about 6000 m. per day and 250 m. per hour. (W. F. D.)

_Satellites of Jupiter._

Jupiter is attended by eight known satellites, resolvable as regards their visibility into two widely different classes. Four satellites were discovered by Galileo and were the only ones known until 1892. In September of that year E. E. Barnard, at the Lick Observatory, discovered a fifth extremely faint satellite, performing a revolution in somewhat less than twelve hours. In 1904 two yet fainter satellites, far outside the other five, were photographically discovered by C. D. Perrine at the Lick Observatory. The eighth satellite was discovered by P. J. Melotte of Greenwich on the 28th of February 1908. It is of the 17th magnitude and appears to be very distant from Jupiter; a re-observation on the 16th of January 1909 proved it to be retrograde, and to have a very eccentric orbit. These bodies are usually numbered in the order of their discovery, the nearest to the sun being V. In apparent brightness each of the four Galilean satellites may be roughly classed as of the sixth magnitude; they would therefore be visible to a keen eye if the brilliancy of the planet did not obscure them. Some observers profess to have seen one or more of these bodies with the naked eye notwithstanding this drawback, but the evidence can scarcely be regarded as conclusive. It does not however seem unlikely that the third, which is the brightest, might be visible when in conjunction with one of the others.

Under good conditions and sufficient telescopic power the satellites are visible as disks, and not mere points of light. Measures of the apparent diameter of objects so faint are, however, difficult and uncertain. The results for the Galilean satellites range between 0´´.9 and 1´´.5, corresponding to diameters of between 3000 and 5000 kilometres. The smallest is therefore about the size of our moon. Satellite I. has been found to exhibit marked variations in its brightness and aspect, but the law governing them has not been satisfactorily worked out. It seems probable that one hemisphere of this satellite is brighter than the other, or that there is a large dark region upon it. A revolution on its axis corresponding with that of the orbital revolution around the planet has also been suspected, but is not yet established. Variations of light somewhat similar, but less in amount, have been noticed in the second and third satellites.

The most interesting and easily observed phenomena of these bodies are their eclipses and their transits across the disk of Jupiter. The four inner satellites pass through the shadow of Jupiter at every superior conjunction, and across his disk at every inferior conjunction. The outer Galilean satellite does the same when the conjunctions are not too near the line of nodes of the satellites' orbit. When most distant from the nodes, the satellites pass above or below the shadow and below or above the disk. These phenomena for the four Galilean satellites are predicted in the nautical almanacs.

When one of the four Galilean satellites is in transit across the disk of Jupiter it can generally be seen projected on the face of the planet. It is commonly brighter than Jupiter when it first enters upon the limb but sometimes darker near the centre of the disk. This is owing to the fact that the planet is much darker at the limb. During these transits the shadow of the satellites can also be seen projected on the planet as a dark point.

The theories of the motion of these bodies form one of the more
interesting problems of celestial mechanics. Owing to the great
ellipticity of Jupiter, growing out of his rapid rotation, the
influence of this ellipticity upon the motions of the five inner
satellites is much greater than that of the sun, or of the satellites
on each other. The inclination of the orbits to the equator of Jupiter
is quite small and almost constant, and the motion of each node is
nearly uniform around the plane of the planet's equator.

The most marked feature of these bodies is a relation between the mean
longitudes of Satellites I., II. and III. The mean longitude of I.
plus twice that of III. minus three times that of II. is constantly
near to 180°. It follows that the same relations subsist among the
mean motions. The cause of this was pointed out by Laplace. If we put
L1 L2 and L3 for the mean longitudes, and define an angle U as
follows:--

U = L1 - 3 L2 + 2 L3.

it was shown mathematically by Laplace that if the longitudes and mean
motions were such that the angle U differed a little from 180°, there
was a minute residual force arising from the mutual actions of the
several bodies tending to bring this angle towards the value 180°.
Consequently, if the mean motions were such that this angle increased
only with great slowness, it would after a certain period tend back
toward the value 180°, and then beyond it, exactly as a pendulum drawn
out of the perpendicular oscillates towards and beyond it. Thus an
oscillation would be engendered in virtue of which the angle would
oscillate very slowly on each side of the central value. Computation
of the mean longitude from observations has indicated that the angle
does differ from 180°, but it is not certain whether this deviation is
greater than the possible result of the errors of observation. However
this may be, the existence of the libration, and its period if it does
exist, are still unknown.

The following are the principal elements of the orbits of the five
inner satellites, arranged in the order of distance from Jupiter. The
mean longitudes are for 1891, 20th of October, G.M.T., and are
referred to the equinox of the epoch, 1891, 2nd of October:--

+--------------------+-----------+--------------+--------------+-----------+-------------+
| Satellite | V. | I. | II. | III. | IV. |
+--------------------+-----------+--------------+--------------+-----------+-------------+
| Mean Long. | 264°.29 | 313°.7193 | 39°.1187 | 171°.2448 | 62°.2000 |
| Synodic Period |11 h. 58 m.|1 d. 18 h. .48| 3d. 13h. .30|7d. 3h. .99|16d. 18m. .09|
| Mean Distance |106,400 m. | 260,000 m. | 414,000 m. | 661,000 m.| 1,162,000 m.|
| Mass ÷ Mass of Jup.| (?) | .00002831 | .00002324 | .00008125 | .00002149 |
| Stellar Mag. | 13 | 6.0 | 6.1 | 5.6 | 6.6 |
+--------------------+-----------+--------------+--------------+-----------+-------------+

The following numbers relating to the planet itself have been supplied
mostly by Professor Hermann Struve.

Filar Mic. Heliom.

Equatorial diameter of Jupiter (Dist. 5.2028) 38´´.50 37´´.50
Polar diameter of Jupiter 36´´.02 35´´.23
Ellipticity 1 ÷ 15.5 1 ÷ 16.5
Theoretical ellipticity from motion of
900´´ in the pericentreof Sat. V 1 ÷ 15.3
Centrifugal force ÷ gravity at equator 0.0900
Mass of Jupiter ÷ Mass of Sun, now used in tables 1 ÷ 1047.34
Inclination of planet's equator to ecliptic 2° 9´.07 + 0.006t
" " " " orbit 3° 4´.80
Long. of Node of equator on ecliptic 336° 21´.47 + 0´.762t
" " " " orbit 135°25´.81 + 0.729t

The longitudes are referred to the mean terrestrial equinox, and t is
the time in years from 1900.0.

For the elements of Jupiter's orbit, see SOLAR SYSTEM; and for
physical constants, see PLANET. (S. N.)

JUR (DIUR), the Dinka name for a tribe of negroes of the upper Nile valley, whose real name is Luoh, or Lwo. They appear to be immigrants, and tradition places their home in the south; they now occupy a district of the Bahr-el-Ghazal between the Bongo and Dinka tribes. Of a reddish black colour, fairer than the Dinka, they are well proportioned, with the hair short. Tattooing is not common, but when found is similar to that of the Dinka; they pierce the ears and nose, and in addition to the ornaments found among the Dinka (q.v.) wear a series of iron rings on the forearm covering it from wrist to elbow. They are mainly agricultural, but hunt and fish to a considerable extent; they are also skilful smiths, smelting their own iron, of which they supply quantities to the Dinka. They are a prosperous tribe and in consequence spinsters are unknown among them. Their chief currency is spears and hoe-blades, and cowrie shells are used in the purchase of wives. Their chief weapons are spears and bows.

See G. Schweinfurth, _The Heart of Africa: Travels 1868-1871_, trans.
G. E. E. Frewer (2nd ed., 1874); W. Junker, _Travels in Africa_ (Eng.
ed., 1890-1892).

JURA, a department of France, on the eastern frontier, formed from the southern portion of the old province of Franche-Comté. It is bounded N by the department of Haute-Saône, N.E. by Doubs, E. by Switzerland, S. by Ain, and W. by Saône-et-Loire and Côte d'Or. Pop. (1906), 257,725. Area, 1951 sq. m. Jura comprises four distinct zones with a general direction from north to south. In the S.E. lie high eastern chains of the central Jura, containing the Crêt Pela (4915 ft.), the highest point in the department. More to the west there is a chain of forest-clad plateaus bordered on the E. by the river Ain. Westward of these runs a range of hills, the slopes of which are covered with vineyards. The north-west region of the department is occupied by a plain which includes the fertile Finage, the northern portion of the Bresse, and is traversed by the Doubs and its left affluent the Loue, between which lies the fine forest of Chaux, 76 sq. m. in area. Jura falls almost wholly within the basin of the Rhone. Besides those mentioned, the chief rivers are the Valouze and the Bienne, which water the south of the department. There are several lakes, the largest of which is that of Chalin, about 12 m. E. of Lons-le-Saunier. The climate is, on the whole, cold; the temperature is subject to sudden and violent changes, and among the mountains winter sometimes lingers for eight months. The rainfall is much above the average of France.

Jura is an agricultural department: wheat, oats, maize and barley are the chief cereals, the culture of potatoes and rape being also of importance. Vines are grown mainly in the cantons of Arbois, Poligny, Salins and Voiteur. Woodlands occupy about a fifth of the area: the oak, hornbeam and beech, and, in the mountains, the spruce and fir, are the principal varieties. Natural pasture is abundant on the mountains. Forests, gorges, torrents and cascades are characteristic features of the scenery. Its minerals include iron and salt and there are stone-quarries. Peat is also worked. Lons-le-Saunier and Salins have mineral springs. Industries include the manufacture of Gruyère, Septmoncel and other cheeses (made in co-operative cheese factories or _fruitières_), metal founding and forging, saw-milling, flour-milling, the cutting of precious stones (at Septmoncel and elsewhere), the manufacture of nails, tools and other iron goods, paper, leather, brier-pipes, toys and fancy wooden-ware and basket-work. The making of clocks, watches, spectacles and measures, which are largely exported, employs much labour in and around Morez. Imports consist of grain, cattle, wine, leaf-copper, horn, ivory, fancy-wood; exports of manufactured articles, wine, cheese, stone, timber and salt. The department is served chiefly by the Paris-Lyon-Méditerranée railway, the main line from Paris to Neuchâtel traversing its northern region. The canal from the Rhone to the Rhine, which utilizes the channel of the Doubs over portions of its course, traverses it for 25 m. Lons-le-Saunier is the chief town of Jura, which embraces four arrondissements named after the towns of Lons-le-Saunier, Dôle, Poligny and St Claude, with 32 cantons and 584 communes. The department forms the diocese of St Claude and part of the ecclesiastical province of Besançon; it comes within the region of the VIIth army corps and the educational circumscription (académie) of Besançon, where is its court of appeal. Lons-le-Saunier, Dôle, Arbois, Poligny, St Claude and Salins, the more noteworthy towns, receive separate notices. At Baume-les-Messieurs, 8 m. N.E. of Lons-le-Saunier, there is an ancient abbey with a fine church of the 12th century.

JURA ("deer island"), an island of the inner Hebrides, the fourth largest of the group, on the west coast of Argyllshire, Scotland. Pop. (1901), 560. On the N. it is separated from the island of Scarba by the whirlpool of Corrievreckan, caused by the rush of the tides, often running over 13 m. an hour, and sometimes accelerated by gales, on the E. from the mainland by the sound of Jura, and on the S. and S.W. from Islay by the sound of Islay. At Kinuachdrach there is a ferry to Aird in Lorne, in Argyllshire, and at Faolin there is a ferry to Port Askaig in Islay. Its area is about 160 sq. m., the greatest length is about 27 m., and the breadth varies from 2 m. to 8 m. The surface is mountainous and the island is the most rugged of the Hebrides. A chain of hills culminating in the Paps of Jura--Beinn-an-Oir (2571 ft.) and Beinn Chaolais (2407 ft.)--runs the whole length of the island, interrupted only by Tarbert loch, an arm of the sea, which forms an indentation nearly 6 m. deep and almost cuts the island in two. Jura derived its name from the red deer which once abounded on it. Cattle and sheep are raised; oats, barley and potatoes are cultivated along the eastern shore, and there is some fishing. Granite is quarried and silicious sand, employed in glass-making is found. The parish of Jura comprises the islands of Balnahua, Fladda, Garvelloch, Jura, Lunga, Scarba and Skervuile.

JURA, a range which may be roughly described as the block of mountains rising between the Rhine and the Rhone, and forming the frontier between France and Switzerland. The gorges by which these two rivers force their way to the plains cut off the Jura from the Swabian and Franconian ranges to the north and those of Dauphiné to the south. But in very early days, before these gorges had been carved out, there were no openings in the Jura at all, and even now its three chief rivers--the Doubs, the Loue and the Ain--flow down the western slope, which is both much longer and but half as steep as the eastern. Some geographers extend the name Jura to the Swabian and Franconian ranges between the Danube and the Neckar and the Main; but, though these are similar in point of composition and direction to the range to the south, it is most convenient to limit the name to the mountain ridges lying between France and Switzerland, and this narrower sense will be adopted here.

The Jura has been aptly described as a huge plateau about 156 m. long and 38 m. broad, hewn into an oblong shape, and raised by internal forces to an average height of from 1950 to 2600 ft. above the surrounding plains. The shock by which it was raised and the vibration caused by the elevation of the great chain of the Alps, produced many transverse gorges or "cluses," while on the plateaus between these subaerial agencies have exercised their ordinary influence.

Geologically the Jura Mountains belong to the Alpine system; and the same forces which crumpled and tore the strata of the one produced the folds and faults in the other. Both chains owe their origin to the mass of crystalline and unyielding rock which forms the central plateau of France, the Vosges and the Black Forest, and which, between the Vosges and the central plateau, lies at no great depth beneath the surface. Against this mass the more yielding strata which lay to the south and west were crushed and folded, and the Alps and the Jura were carved from the ridges which were raised. But the folding decreases in intensity towards the north; the folding in the Alps is much more violent than the folding in the Jura, and in the Jura itself the folding is most marked along its southern flanks.

The Jura is composed chiefly of Jurassic rocks--it is from this chain that the Jurassic system derives its name--but Triassic, Cretaceous and Tertiary beds take part in its formation. It may be divided into three zones which run parallel to the length of the chain and differ from one another in their structure. The innermost zone, which rises directly from the plain of Switzerland, is the _folded Jura_ (_Jura plissé, Kettenjura_), formed of narrow parallel undulations which diminish in intensity towards the French border. This is followed by the _Jura plateau_ (_Jura tabulaire_, _Tafeljura_), in which the beds are approximately horizontal but are broken up into blocks by fractures or faults. Finally, along its western face there is a zone of numerous dislocations, and the range descends abruptly to the plain of the Saône. This is the _Région du vignoble_ and is well shown at Arbois.

Owing to the convergence of the faults which bound it, the plateau zone decreases in width towards the south, while towards the north it forms a large proportion of the chain. The folded zone is more constant. Along its inner margin the folds are frequently overthrown, leaning towards France, but elsewhere they are simple anticlinals and synclinals, parallel to the length of the chain, and as a rule there is a remarkable freedom from dislocations of any importance, except towards Neuchâtel and Bienne.

The countless blocks of gneiss, granite and other crystalline formations which are found in such numbers on the slopes of the Jura, and go by the name of "erratic blocks" (of which the best known instance--the Pierre à Bot--is 40 ft. in diameter, and rests on the side of a hill 800 ft. above the Lake of Neuchâtel), have been transported thither from the Alps by ancient glaciers, which have left their mark on the Jura range itself in the shape of striations and moraines.

The general direction of the chain is from north-east to south-west, but a careful study reveals the fact that there were in reality two main lines of upheaval, viz. north to south and east to west, the former best seen in the southern part of the range and the latter in the northern; and it was by the union of these two forces that the lines north-east to south-west (seen in the greater part of the chain), and north-west to south-east (seen in the Villebois range at the south-west extremity of the chain), were produced. This is best realized if we take Besançon as a centre; to the north the ridges run east and west, to the south, north and south, while to the east the direction is north-east to south-west.

Before considering the topography of the interior of the Jura, it may
be convenient to take a brief survey of its outer slopes.

1. The _northern face_ dominates on one side the famous "Trouée" (or
Trench) of Belfort, one of the great geographical centres of Europe,
whence routes run north down the Rhine to the North Sea, south-east to
the Danube basin and Black Sea, and south-west into France, and so to
the Mediterranean basin. It is now so strongly fortified that it
becomes a question of great strategical importance to prevent its
being turned by means of the great central plateau of the Jura, which,
as we shall see, is a network of roads and railways. On the other side
it overhangs the "Trouée" of the Black Forest towns on the Rhine
(Rheinfelden, Säckingen, Laufenburg and Waldshut), through which the
central plain of Switzerland is easily gained. On this north slope two
openings offer routes into the interior of the chain--the valley of
the Doubs belonging to France, and the valley of the Birse belonging
to Switzerland. Belfort is the military, Mülhausen the industrial, and
Basel the commercial centre of this slope.

2. The _eastern and western faces_ offer many striking parallels. The
plains through which flow the Aar and the Saône have each been the bed
of an ancient lake, traces of which remain in the lakes of Neuchâtel,
Bienne and Morat. The west face runs mainly north and south like its
great river, and for a similar reason the east face runs north-east to
south-west. Again, both slopes are pierced by many transverse gorges
or "cluses" (due to fracture and not to erosion), by which access is
gained to the great central plateau of Pontarlier, though these are
seen more plainly on the east face than on the west; thus the gorges
at the exit from which Lons-le-Saunier, Poligny, Arbois and Salins are
built balance those of the Suze, of the Val de Ruz, of the Val de
Travers, and of the Val d'Orbe, though on the east face there is but
one city which commands all these important routes--Neuchâtel. This
town is thus marked out by nature as a great military and industrial
centre, just as is Besançon on the west, which has besides to defend
the route from Belfort down the Doubs. These easy means of
communicating with the Free County of Burgundy or Franche-Comté
account for the fact that the dialect of Neuchâtel is Burgundian, and
that it was held generally by Burgundian nobles, though most of the
country near it was in the hands of the house of Savoy until gradually
annexed by Bern. The Chasseron (5286 ft.) is the central point of the
eastern face, commanding the two great railways which join Neuchâtel
and Pontarlier. This ridge is in a certain sense parallel to the
valley of the Loue on the west face, which flows into the Doubs a
little to the south of Dôle, the only important town of the central
portion of the Saône basin. The Chasseron is wholly Swiss, as are the
lower summits of the Chasseral (5279 ft.), the Mont Suchet (5220 ft.),
the Aiguille de Baulmes (5128 ft.), the Dent de Vaulion (4879 ft.),
the Weissenstein (4223 ft.), and the Chaumont (3845 ft.), the two
last-named points being probably the best-known points in the Jura, as
they are accessible by carriage road from Soleure and Neuchâtel
respectively. South of the Orbe valley the east face becomes a rocky
wall which is crowned by all the highest summits (the first and second
Swiss, the rest French) of the chain--the Mont Tendre (5512 ft.), the
Dôle (5505 ft.), the Reculet (5643 ft.), the Crêt de la Neige (5653
ft.) and the Grand Crédo (5328 ft.), the uniformity of level being as
striking as on the west edge of the Jura, though there the absolute
height is far less. The position of the Dôle is similar to that of the
Chasseron, as along the sides of it run the great roads of the Col de
St Cergues (3973 ft.) and the Col de la Faucille (4341 ft.), the
latter leading through the Vallée des Dappes, which was divided in
1862 between France and Switzerland, after many negotiations. The
height of these roads shows that they are passages across the chain,
rather than through natural depressions.

3. The _southern face_ is supported by two great pillars--on the east
by the Grand Crédo and on the west by the ridge of Revermont (2529
ft.) above Bourg en Bresse; between these a huge bastion (the district
of _Bugey_) stretches away to the south, forcing the Rhone to make a
long détour. On the two sides of this bastion the plains in which
Ambérieu and Culoz stand balance one another, and are the meeting
points of the routes which cut through the bastion by means of deep
gorges. On the eastern side this great wedge is steep and rugged,
ending in the Grand Colombier (5033 ft.) above Culoz, and it sinks on
the western side to the valley of the Ain, the district of Bresse, and
the plateau of Dombes. The junction of the Ain and the Surand at Pont
d'Ain on the west balances that of the Valserine and the Rhone at
Bellegarde on the east.

The Jura thus dominates on the north one of the great highways of
Europe, on the east and west divides the valleys of the Saône and the
Aar, and stretches out to the south so as nearly to join hands with
the great mass of the Dauphiné Alps. It therefore commands the routes
from France into Germany, Switzerland and Italy, and hence its
enormous historical importance.

Let us now examine the topography of the interior of the range. This
naturally falls into three divisions, each traversed by one of the
three great rivers of the Jura--the Doubs, the Loue and the Ain.

1. In the _northern division_ it is the east and west line which
prevails--the Lomont, the Mont Terrible, the defile of the Doubs from
St Ursanne to St Hippolyte, and the "Trouée" of the Black Forest
towns. It thus bars access to the central plateau from the north, and
this natural wall does away with the necessity of artificial
fortifications. This division falls again into two distinct portions.

(a) The first is the _part east of the deep gorge of the Doubs_ after
it turns south at St Hippolyte; it is thus quite cut off on this side,
and is naturally Swiss territory. It includes the basin of the river
Birse, and the great plateau between the Doubs and the Aar, on which,
at an average height of 2600 ft., are situated a number of towns, one
of the most striking features of the Jura. These include Le Locle
(q.v.) and La Chaux de Fonds (q.v.), and are mainly occupied with
watch-making, an industry which does not require bulky machinery, and
is therefore well fitted for a mountain district.

(b) _The part west of the "cluse" of the Doubs_: of this, the district
east of the river Dessoubre, isolated in the interior of the range
(unlike the Le Locle plateau), is called the Haute Montagne, and is
given up to cheese-making, curing of hams, saw-mills, &c. But little
watch-making is carried on there, Besançon being the chief French
centre of this industry, and being connected with Geneva by a chain of
places similarly occupied, which fringe the west plateau of the Jura.
The part west of the Dessoubre, or the Moyenne Montagne, a huge
plateau north of the Loue, is more especially devoted to agriculture,
while along its north edge metal-working and manufacture of hardware
are carried on, particularly at Besançon and Audincourt.

2. The _central division_ is remarkable for being without the deep
gorges which are found so frequently in other parts of the range. It
consists of the basin of which Pontarlier is the centre, through
notches in the rim of which routes converge from every direction; this
is the great characteristic of the middle region of the Jura. Hence
its immense strategical and commercial importance. On the north-east
roads run to Morteau and Le Locle, on the north-west to Besançon, on
the west to Salins, on the south-west to Dôle and Lons-le-Saunier, on
the east to the Swiss plain. The Pontarlier plateau is nearly
horizontal, the slight indentations in it being due to erosion, e.g.
by the river Drugeon. The keys to this important plateau are to the
east the Fort de Joux, under the walls of which meet the two lines of
railway from Neuchâtel, and to the west Salins, the meeting place of
the routes from the Col de la Faucille, from Besançon, and from the
French plain.

The Ain rises on the south edge of this plateau, and on a lower shelf
or step, which it waters, are situated two points of great military
importance--Nozeroy and Champagnole. The latter is specially
important, since the road leading thence to Geneva traverses one after
another, not far from their head, the chief valleys which run down
into the South Jura, and thus commands the southern routes as well as
those by St Cergues and the Col de la Faucille from the Geneva region,
and a branch route along the Orbe river from Jougne. The fort of Les
Rousses, near the foot of the Dôle, serves as an advanced post to
Champagnole, just as the Fort de Joux does to Pontarlier.

The above sketch will serve to show the character of the central Jura
as the meeting place of routes from all sides, and the importance to
France of its being strongly fortified, lest an enemy approaching from
the north-east should try to turn the fortresses of the "Trouée de
Belfort." It is in the western part of the central Jura that the north
and south lines first appear strongly marked. There are said to be in
this district no less than fifteen ridges running parallel to each
other, and it is these which force the Loue to the north, and thereby
occasion its very eccentric course. The cultivation of wormwood
wherewith to make the tonic "absinthe" has its headquarters at
Pontarlier.

3. The _southern division_ is by far the most complicated and
entangled part of the Jura. The lofty ridge which bounds it to the
east forces all its drainage to the west, and the result is a number
of valleys of erosion (of which that of the Ain is the chief
instance), quite distinct from the natural "cluses" or fissures of
those of the Doubs and of the Loue. Another point of interest is the
number of roads which intersect it, despite its extreme irregularity.
This is due to the great "cluses" of Nantua and Virieu, which traverse
it from east to west. The north and south line is very clearly seen in
the eastern part of this division; the north-east and south-west is
entirely wanting, but in the Villebois range south of Ambérieu we have
the principal example of the north-west to south-east line. The
plateaus west of the Ain are cut through by the valleys of the Valouse
and of the Surand, and like all the lowest terraces on the west slope
do not possess any considerable towns. The Ain receives three
tributaries from the east:--

(a) The Bienne, which flows from the fort of Les Rousses by St Claude,
the industrial centre of the south Jura, famous for the manufacture of
wooden toys, owing to the large quantity of boxwood in the
neighbourhood. Septmoncel is busied with cutting of gems, and Morez
with watch and spectacle making. Cut off to the east by the great
chain, the industrial prosperity of this valley is of recent origin.

(b) The Oignin, which flows from south to north. It receives the
drainage of the lake of Nantua, a town noted for combs and silk
weaving, and which communicates by the "cluse" of the Lac de Silan
with the Valserine valley, and so with the Rhone at Bellegarde, and
again with the various routes which meet under the walls of the fort
of Les Rousses, while by the Val Romey and the Séran Culoz is easily
gained.

(c) The Albarine, connected with Culoz by the "cluse" of Virieu, and
by the Furan flowing south with Belley, the capital of the district of
Bugey (the old name for the South Jura).

The "cluses" of Nantua and Virieu are now both traversed by important
railways; and it is even truer than of old that the keys of the south
Jura are Lyons and Geneva. But of course the strategic importance of
these gorges is less than appears at first sight, because they can be
turned by following the Rhone in its great bend to the south.

The range is mentioned by Caesar (_Bell. Gall._ i. 2-3, 6 (1), and 8 (1)), Strabo (iv. 3, 4, and 6, 11), Pliny (iii. 31; iv. 105; xvi. 197) and Ptolemy (ii. ix. 5), its name being a word which appears under many forms (e.g. Joux, Jorat, Jorasse, Juriens), and is a synonym for a wood or forest. The German name is Leberberg, _Leber_ being a provincial word for a hill.

Politically the Jura is French (departments of the Doubs, Jura and Ain) and Swiss (parts of the cantons of Geneva, Vaud, Neuchâtel, Bern, Soleure and Basel); but at its north extremity it takes in a small bit of Alsace (Pfirt or Ferrette). In the middle ages the southern, western and northern sides were parcelled out into a number of districts, all of which were gradually absorbed by the French crown, viz., Gex, Val Romey, Bresse and Bugey (exchanged in 1601 by Savoy for the marquisate of Saluzzo), Franche-Comté, or the Free County of Burgundy, an imperial fief till annexed in 1674, the county of Montbéliard (Mömpelgard) acquired in 1793, and the county of Ferrette (French 1648-1871). The northern part of the eastern side was held till 1792 (part till 1797) by the bishop of Basel as a fief of the empire, and then belonged to France till 1814, but was given to Bern in 1815 (as a recompense for its loss of Vaud), and now forms the Bernese Jura, a French-speaking district. The centre of the eastern slope formed the principality of Neuchâtel (q.v.) and the county of Valangin, which were generally held by Burgundian nobles, came by succession to the kings of Prussia in 1707, and were formed into a Swiss canton in 1815, though they did not become free from formal Prussian claims until 1857. The southern part of the eastern slope originally belonged to the house of Savoy, but was conquered bit by bit by Bern, which was forced in 1815 to accept its subject district Vaud as a colleague and equal in the Swiss Confederation. It was Charles the Bold's defeats at Grandson and Morat which led to the annexation by the confederates of these portions of Savoyard territory.

AUTHORITIES.--E. F. Berlioux, _Le Jura_ (Paris, 1880); F. Machacek,
_Der Schweizer Jura_ (Gotha, 1905); A. Magnin, _Les lacs du Jura_
(Paris, 1895); J. Zimmerli, "Die Sprachgrenze im Jura" (vol. i. of his
_Die Deutsch-französische Sprachgrenze in der Schweiz_ (Basel, 1891).
For the French slope see Joanne's large _Itinéraire_ to the Jura, and
the smaller volumes relating to the departments of the Ain, Doubs and
Jura, in his _Géographies départementales_. For the Swiss slope see 3
vols. in the series of the _Guides Monod_ (Geneva); A. Monnier, _La
Chaux de Fonds et le Haut-Jura Neuchâtelois_; J. Monod, _Le Jura
Bernois_; and E. J. P. de la Harpe, _Le Jura Vaudois_.
(W. A. B. C.)

JURASSIC, in geology, the middle period of the Mesozoic era, that is to say, succeeding the Triassic and preceding the Cretaceous periods. The name Jurassic (French _jurassique_; German _Juraformation_ or _Jura_) was first employed by A. Brongniart and A. von Humboldt for the rocks of this age in the western Jura mountains of Switzerland, where they are well developed. It was in England, however, that they were first studied by William Smith, in whose hands they were made to lay the foundations of stratigraphical geology. The names adopted by him for the subdivisions he traced across the country have passed into universal use, and though some of them are uncouth English provincial names, they are as familiar to the geologists of France, Switzerland and Germany as to those of England. During the following three decades Smith's work was elaborated by W. D. Conybeare and W. Phillips. The Jurassic rocks of fossils of the European continent were described by d'Orbigny, 1840-1846; by L. von Buch, 1839; by F. A. Quenstedt, 1843-1888; by A. Oppel, 1856-1858; and since then by many other workers: E. Benecke, E. Hébert, W. Waagen, and others. The study of Jurassic rocks has continued to attract the attention of geologists, partly because the bedding is so well defined and regular--the strata are little disturbed anywhere outside the Swiss Jura and the Alps--and partly because the fossils are numerous and usually well-preserved. The result has been that no other system of rocks has been so carefully examined throughout its entire thickness; many "zones" have been established by means of the fossils--principally by ammonites--and these zones are not restricted to limited districts, but many of them hold good over wide areas. Oppel distinguished no fewer than thirty-three zonal horizons, and since then many more sub-zonal divisions have been noted locally.

The existence of _faunal regions_ in Jurassic times was first pointed out by J. Marcou; later M. Neumayr greatly extended observations in this direction. According to Neumayr, three distinct geographical regions of deposit can be made out among the Jurassic rocks of Europe: (1) The Mediterranean province, embracing the Pyrenees, Alps and Carpathians, with all the tracts lying to the south. One of the biological characters of this area was the great abundance of ammonites belonging to the groups of _Heterophylli_ (_Phylloceras_) and _Fimbriati_ (_Lytoceras_). (2) The central European province, comprising the tracts lying to the north of the Alpine ridge, and marked by the comparative rarity of the ammonites just mentioned, which are replaced by others of the groups _Inflati_ (_Aspidoceras_) and _Oppelia_, and by abundant reefs and masses of coral. (3) The boreal or Russian province, comprising the middle and north of Russia, Spitzbergen and Greenland. The life in this area was much less varied than in the others, showing that in Jurassic times there was a perceptible diminution of temperature towards the north. The ammonites of the more southern tracts here disappear, together with the corals.

The cause of these faunal regions Neumayr attributed to climatic belts--such as exist to-day--and in part, at least, he was probably correct. It should be borne in mind, however, that although Neumayr was able to trace a broad, warm belt, some 60° in width, right round the earth, with a narrower mild belt to the north and an arctic or boreal belt beyond, and certain indications of a repetition of the climatic zones on the southern side of the thermal equator, more recent discoveries of fossils seem to show that other influences must have been at work in determining their distribution; in short, the identity of the Neumayrian climatic boundaries becomes increasingly obscured by the advance of our knowledge.

The Jurassic period was marked by a great extension of the sea, which commenced after the close of the Trias and reached its maximum during the Callovian and Oxfordian stages; consequently, the Middle Jurassic rocks are much more widely spread than the Lias. In Europe and elsewhere Triassic beds pass gradually up into the Jurassic, so that there is difficulty sometimes in agreement as to the best line for the base of the latter; similarly at the top of the system there is a passage from the Jurassic to the Cretaceous rocks (Alps).

Towards the close of the period elevation began in certain regions; thus, in America, the Sierras, Cascade Mountains, Klamath Mountains, and Humboldt Range probably began to emerge. In England the estuarine Portlandian resulted partly from elevation, but in the Alps marine conditions steadily persisted (in the Tithonian stage). There appears to have been very little crustal disturbance or volcanic activity; tuffs are known in Argentina and California; volcanic rocks of this age occur also in Skye and Mull.

The rocks of the Jurassic system present great petrological diversity. In England the name "Oolites" was given to the middle and higher members of the system on account of the prevalence of oolitic structure in the limestones and ironstones; the same character is a common feature in the rocks of northern Europe and elsewhere, but it must not be overlooked that clays and sandstones together bulk more largely in the aggregate than the oolites. The thickness of Jurassic rocks in England is 4000 to 5000 ft., and in Germany 2000 to 3000 ft. Most of the rocks represent the deposits of shallow seas, but estuarine conditions and land deposits occur as in the Purbeck beds of Dorset and the coals of Yorkshire. Coal is a very important feature among Jurassic rocks, particularly in the Liassic division; it is found in Hungary, where there are twenty-five workable beds; in Persia, Turkestan, Caucasus, south Siberia, China, Japan, Further India, New Zealand and in many of the Pacific Islands.

Being shallow water formations, petrological changes come in rapidly as many of the beds are traced out; sandstones pass laterally into clays, and the latter into limestones, and so on, but a reliable guide to the classification and correlation is found in the fossil contents of the rocks. In the accompanying table a list is given of some of the zonal fossils which regularly occur in the order indicated; other forms are known that are equally useful. It will be noticed that while there is general agreement as to the order in which the zonal forms occur, the line of division between one formation and another is liable to vary according to factors in the personal equation of the authors.

The Jurassic formations stretch across England in a varying band from the mouth of the Tees to the coast of Dorsetshire. They consist of harder sandstones and limestones interstratified with softer clays and shales. Hence they give rise to a characteristic type of scenery--the more durable beds standing out as long ridges, sometimes even with low cliffs, while the clays underlie the level spaces between.

Jurassic rocks cover a vast area in Central Europe. They rise from
under the Cretaceous formations in the north-east of France, whence
they range southwards down the valleys of the Saône and Rhone to the
Mediterranean. They appear as a broken border round the old
crystalline nucleus of Auvergne. Eastwards they range through the Jura
Mountains up to the high grounds of Bohemia. They appear in the outer
chains of the Alps on both sides, and on the south they rise along the
centre of the Apennines, and here and there over the Spanish
Peninsula. Covered by more recent formations they underlie the great
plain of northern Germany, whence they range eastwards and occupy
large tracts in central and eastern Russia.

Lower Jurassic rocks are absent from much of northern Russia, the
stages represented being the Callovian, Oxfordian and Volgian (of
Professor S. Nikitin); the fauna differs considerably from that of
western Europe, and the marine equivalents of the Purbeck beds are
found in this region. In south Russia, the Crimea and Caucasus, Lias
and Lower Jurassic rocks are present. In the Alps, the Lower Jurassic
rocks are intimately associated with the underlying Triassic
formations, and resemble them in consisting largely of reddish
limestones and marbles; the ammonites in this region differ in certain
respects from those of western and central Europe. The Oxfordian,
Callovian, Corallian and Astartian stages are also present. The Upper
Jurassic is mainly represented by a uniform series of limestones, with
a peculiar and characteristic fauna, to which Oppel gave the name
"Tithonian." This includes most of the horizons from Kimeridgian to
Cretaceous; it is developed on the southern flanks of the Alps,
Carpathians, Apennines, as well as in south France and other parts of
the Mediterranean basin. A characteristic formation on this horizon is
the "Diphya limestone," so-called from the fossil _Terebratula diphya_
(_Pygope janitor_) seen in the well-known escarpments (_Hochgebirge
Kalk_). Above the Diphya limestone comes the Stramberg limestone
(Stramberg in Moravia), with "Aptychus" beds and coral reefs. The
rocks of the Mediterranean basin are on the whole more calcareous than
those of corresponding age in north-west Europe; thus the Lias is
represented by 1500 ft. of white crystalline limestone in Calabria and
a similar rock occurs in Sicily, Bosnia, Epirus, Corfu; in Spain the
Liassic strata are frequently dolomitic; in the Apennines they are
variegated limestones and marls. The Higher Jurassic beds of Portugal
show traces of the proximity of land in the abundant plant remains
that are found in them. In Scania the Lias succeeds the Rhaetic beds
in a regular manner, and Jurassic rocks have been traced northward
well within the polar circle; they are known in the Lofoten Isles,
Spitzbergen, east Greenland, King Charles's Island, Cape Stewart in
Scoresby Sound, Grinnell Land, Prince Patrick Land, Bathurst and
Exmouth Island; in many cases the fossils denote a climate
considerably milder than now obtains in these latitudes.

In the American continent Jurassic rocks are not well developed.
Marine Lower and Middle Jurassic beds occur on the Pacific coast
(California and Oregon), and in Wyoming, the Dakotas, Colorado, east
Mexico and Texas. Above the marine beds in the interior are brackish
and fresh-water deposits, the Morrison and Como beds (Atlantosaurus
and Baptanodon beds of Marsh). Later Jurassic rocks are found in
northern British Columbia and perhaps in Alaska, Wyoming, Utah,
Montana, Colorado, the Dakotas, &c. In California some of the
gold-bearing, metamorphic slates are of this age. Marine Jurassic
rocks have not been clearly identified on the Atlantic side of
America. The Patuxent and Arundel formations (non-marine) are
doubtfully referred to this period. Lower and Middle Jurassic
formations occur in Argentina and Bolivia. Jurassic rocks have been
recognized in Asia, including India, Afghanistan, Persia, Kurdistan,
Asia Minor, the Caspian region, Japan and Borneo. The best marine
development is in Cutch, where the following groups are distinguished
from above downwards: the Umia series = Portlandian and Tithonian of
south Europe, passing upwards into the Neocomian; the Katrol series =
Oxfordian (part) and Kimeridgian; the Chari series = Callovian and
part of the Oxfordian; the Patcham series = Bathonian. In the western
half of the Salt Range and the Himalayas, Spiti shales are the
equivalents of the European Callovian and Kimeridgian. The upper part
of the Gondwana series is not improbably Jurassic. On the African
continent, Liassic strata are found in Algeria, and Bathonian
formations occur in Abyssinia, Somaliland, Cape Colony and western
Madagascar. In Australia the Permo-Carboniferous formations are
succeeded in Queensland and Western Australia by what may be termed
the Jura-Trias, which include the coal-bearing "Ipswich" and "Burrum"
formations of Queensland. In New Zealand there is a thick series of
marine beds with terrestrial plants, the Mataura series in the upper
part of Hutton's Hokanui system. Sir J. Hector included also the
Putakaka series (as Middle Jurassic) and the Flag series with the
Catlin's River and Bastion series below. Jurassic rocks have been
recorded from New Guinea and New Caledonia.

JURASSIC SYSTEM