Chapter XIX: Introduction (2)

MARLSTONE OR MIDDLE LIAS.--The Marlstone (rock bed) is largely quarried on the Avon and Burton Dassett Hills. It forms a good building stone, more or less indurated, of a green or yellow brown colour, sometimes ferruginous. It forms a conspicuous range of hills of moderate height of which Edge Hill is the highest, from which it strikes southward towards Oxfordshire. The plain below is occupied by the underlying division of the Lower Lias. In this county the marlstone contains very few fossils, and those chiefly brachiopodous shells belonging to the genus _Terebratula_. In most cases elsewhere the Marlstone proper, or highest zone, is very fossiliferous, and abounds in marine shells, which are usually well preserved. The sandy beds immediately below are rarely exposed, but crop out in a lane near Bitham House, where as usual they contain many fossils. The inferior clays and marls are not visible except in some brick pits near Fenny Compton and along the line of railway. These are very full of fossils in the zone of _Ammonites Jamiesoni_ and _Ibex_, here nearly one hundred feet thick, and especially at one horizon in a coarse, hard, stony band which contains numerous corals towards the upper part of the cutting, near the station.

LOWER LIAS.--For the most part this formation spreads over the portion of the country on the north-east, east, south-east, south and south-west of Warwick. A remarkably fine section is exposed in the railway cutting near Harbury Station. This portion of the series is also largely quarried at Rugby, and in other places south and south-east of Stratford. The strata consist of beds of blue clay or shale interstratified with beds of blue rubbly and argillaceous limestone, much quarried for hydraulic lime. One good section of the lime-yielding beds occurs at Messrs. Greaves and Lakin’s Quarries at Stockton and Harbury. The lowest zones of the Lias are largely quarried at Wilmcote, and may be seen at the remarkable outlier of Brown’s Wood, near Henley-in-Arden, and at another (Copt Heath), near Knowle. These two last are of special interest, because they shew the lowest beds of the Lias (in connection with and passing into the Rhætics,) resting immediately upon the New Red Marls. The thickness of the Lower Lias in the county is above 600 feet; but only the inferior zones of _Ammonites angulatus_ and _A. planorbis_ are laid open to any great extent. The best sections of the _Lima_ beds (_A. angulatus_ zone) occur in the railway cutting at Harbury, Stockton lime quarries, and the extensive quarries at Newbold near Rugby. Fossils are not very numerous, but the following occur:--_Gryphea incurva_, _Rhynchonella variabilis_, _Ammonites angulatus_, _Pecten_, various species, _Lima gigantea_, and bones and teeth of _Plesiosaurus_ and _Ichthyosaurus_. Fish are comparatively rare, two or three only were found at Harbury and a very few near Rugby.

The higher ground round Wilmcote and Binton is also capped by these Lima beds; but the district is more or less affected by small faults, so that certain beds in one contiguous quarry are absent in another. The lower limestones (insect beds) are largely worked in this locality, and are of much economical value. With the exception of remains of insects and fragments of plants, the fossils are entirely marine, _Ammonites planorbis_ and _A. Johnstoni_, being abundant and characteristic. Crustacea belonging to the genera _Astacus_ and _Eryon_, the latter of great size are not unfrequently met with. Small fishes, _Pholidophorus Stricklandi_, and the larger _Dapedium_ and _Tetragonolepis_ more rarely occur. A fine example of the latter is preserved in the Warwick Museum. The large _Enaliosaurians_ are well represented by some fine specimens of _Ichthyosaurus_ and _Plesiosaurus_; the _P. megacephalus_ in the Museum at Warwick being nearly entire, measuring 14 feet 4 inches in length. Large masses of driftwood and a few fronds of ferns are sometimes met with. But perhaps the most interesting and remarkable fossils are the insects. Twenty-four families and genera were determined more than twenty years ago, since which time many important additions have been made. The _Coleoptera_ and _Neuroptera_ are most numerous. Small beetles are not unfrequently found entire. Among these may be noted the families _Buprestidæ_, _Elateridæ_, _Carabidæ_, and others.

There are also remains of _Orthoptera_, _Homoptera_, _Libellulidæ_, and some _Diptera_. Many of the _Neuroptera_ were evidently of gigantic proportions, but most of the insects were of small size, and like the associated plants, are indicative of a temperate climate. They are most nearly allied to forms which now inhabit North America. There are few extinct or unknown genera among them.

RHÆTIC SERIES.--The highest beds referred to this series consist of certain hard, fine grained limestones, which, from their ordinary white colour, have been termed _White Lias_. They occupy a considerable area south and south-east of Warwick. They constitute a purely local deposit, and are confined for the most part to this county and Somersetshire. They are often close-grained and hard limestones, and make a useful building material and a good lime. Their colour is mostly white, with a yellow tinge, and occasionally pink and grey. Some geologists consider these beds to belong to the “Rhætic Series,” others to the passage beds between the Lias and the latter, while others still class them with the Lias.

The undisputed Rhætic rocks lie between the White Lias and the Triassic Marls. In Warwickshire they are rarely exposed, and then much reduced in bulk, compared with their development in Gloucester and Glamorgan. They may be seen to a limited extent below the White Lias in the railway cutting at Harbury, where a band of yellowish sandstone contains the small bivalved crustacean, _Estheria minuta_; and also at the small outlier of Brown’s Wood, and at Stooper’s Wood, near Wooton Wawen, where this sandstone occurs with inferior shelly limestones and sandy bands, containing the usual Rhætic fossils, _e.g._, _Cardium Rhæticum_, _Avicula contorta_, _Pleurophorus elongatus_, _Pecten valoniensis_, and _Schizodus cloacinus_. The nearest exposure of the Rhætic to Birmingham occurs round the fringe of an outlier of Lower Lias resting on the Upper Red Marl near the village of Knowle. This outlier is about a mile and a half long, by half a mile broad. Its highest beds at Copt Heath contain _Ammonites planorbis_. The beds referred to the Rhætic include a stratum of yellow micaceous sandstone full of _Schizodus cloacinus_, which, though usually in the form of casts, is sharp and well defined. The bone bed, though no where exposed, is probably present in its normal position. A fine section, with numerous characteristic Rhætic fossils was exposed on the railway cutting at Summer Hill, between Stratford and Alcester. Rhætic black shales were passed through at Snitterfield, in making a tunnel in connection with the new reservoir for Stratford.

Glacial and Post Tertiary Deposits.

BY H. W. CROSSKEY, LL.D., F.G.S.

Post Tertiary Deposits are scattered profusely over the district of which Birmingham is the centre, and present many problems of too complicated a character to be discussed in the pages of this guide. It must suffice to indicate a few of their chief exposures and characteristics. The term “_Boulder Clay_” is used in this note to denote a clay shown to be connected with the Glacial epoch, by containing a greater or less number of erratic blocks; and in the employment of the term, no theory regarding the method of formation of the deposit will be implied.

The Post Tertiary deposits of the district may be arranged in the following general order:--

I.--Lower boulder clays.

II.--Middle glacial clays, sands, and gravels.

III.--Upper boulder clays.

IV.--Post glacial clay, sands, and gravels.

The most complete section that has been found is at “California,” near Harborne.

Resting upon the Bunter Sandstone, about 480 feet above the sea level, is a _Lower boulder clay_, containing erratic boulders of slate, felsite, quartzite, intermixed with blocks and stones of local origin. Many of the erratics are angular, and some (especially the slates) are finely striated. The whole deposit is unstratified and compact, and the boulders are roughly pressed together, in every variety of position, without any orderly arrangement. This boulder clay is succeeded by the _Middle Sands and Gravels_ which are irregularly stratified and show false bedding. Fragments of coal occur among the pebbles. The sands and gravels dip rapidly to the S.W., and pass under an Upper boulder clay. _The Upper Boulder Clay_ consists of a compact mass of clay with erratics scattered through it; but the erratics are neither so abundant nor so confusedly pressed together as in the lower bed. Granite has been found, although rarely, associated with the travelled felsites and quartzites, together with a few flints; and local stones and blocks are also mixed up with the clay--the clay itself however largely preponderating and being available for brick making.

The series is capped by a mixture of clay, sand, and gravel in varying proportions, which fills many hollows that have been washed out of the upper clay; and must be regarded as Post Glacial. Taking the general divisions indicated by the California section, attention may be directed to the following illustrative facts and sections. Glacial striæ upon the surface of the rock have been noticed at Weoley Hill Quarry close to California. The removal of a mass of clay, sand, and gravel exposed a distinctly striated surface of hardened Bunter Sandstone. The polished surface dips towards the south west, in which direction the principal striæ run, although there are several cross striæ. The complete section shews (_a_) striated and polished Bunter (altitude 520 feet above sea level); (_b_) thin bed of marl; (_c_) sands and gravels (Middle Glacial); (_d_) clay (Upper Boulder Clay).

A very large number of well-marked and finely smoothed and polished grooves occur upon the blocks of native rock which are strewn over the irregularly shaped mass of basalt constituting Rowley Hill, Worcestershire. Isolated grooved blocks rest upon the surface of the hill, having been carried by external force into their present position; but there is also, at Rowley Hall Quarry, a kind of platform, capping the solid mass of basalt, which is almost entirely composed of blocks with smoothed and grooved surfaces, stiffly imbedded in clay.

The question has been raised whether, since the basalt readily develops joints, these grooves may not be rudimentary joints, or whether disintegration may not have taken place along certain lines which have gradually become grooves. I entertain no doubt, however, of their glacial origin. No other explanation than that the grooves were the work of moving ice can account, I think, for their excellent polish, their frequent parallelism, their adaptation to the hollows and protuberances of the blocks they cross, and their predominant trend from north-east to south-west. The absence of erratics from the boulder clay in which the grooved basaltic blocks are embedded is evidence of local ice action at Rowley Hill. It is notable also that angular blocks of basalt from Rowley Hill have been found in Birmingham, blocks which must have travelled at least six miles.

It is often difficult to decide the precise age of the boulder clays of the district; and whether any individual bed is referred to the upper or the lower series is more dependent upon the glacial theories that may be adopted than upon any observations that can be made in the field. A _Boulder Clay_, of a typical kind, has been exposed at a brickyard, at the bottom of Oak Street, Wolverhampton. This clay contains an extraordinary number and variety of erratic blocks, without question from the Lake District and south of Scotland, a few flints, together with pebbles from the Bunter beds. One of the sides of a boulder of felsite, measuring 11 × 3 × 3ft., is flat and smooth, and covered with parallel striæ. The sands and gravels rising in small hillocks near the Cemetery, and slightly covering the clay of the pit, are probably Middle glacial. A boulder clay, formerly exposed at Icknield Street, Birmingham, while presenting the same physical characteristics as the Wolverhampton clay, differed from it widely in the nature of the embedded erratic blocks. Instead of having travelled from the Lake District or Scotland, a large proportion were derived from rocks that occur _in situ_ at the Berwyn and Arenig Hills. The condition of the New Red Sandstone ridge, against which this boulder clay rested, was remarkable. The sandstone rock was broken up, and large fragments of it were lifted out of their position and thrust into the middle of the drift.

The changes of level which occurred during the glacial epoch are shown by the deposits at Frankley Hill. In the clays and sands cut through by the Halesowen Railway only a few erratics (felsites) were found; but on the summit of the section they are abundant and of large size (_e.g._, 4 × 4 × 2ft.) Professor Bonney, who has examined them, feels certain they must have come from Wales, having seen nothing like them in the Lake District. Their height is nearly 800ft. above the sea level. Were these erratics brought by land ice, the alteration in the physical geography of the country must have been enormous to have enabled a glacier to have moved downwards over this point; were they dropped from icebergs, the land must have been depressed to the extent of at least 900ft., to form a sea in which the bergs could have floated.

Turning to the _Middle Glacial Clays, Sands, and Gravels_, these may be seen more or less developed in almost every section cut through undulating ground; and they are occasionally twisted and contorted. In the immediate neighbourhood of Birmingham they are not fossiliferous.

At Ketley, near Wellington (Shropshire) however, fossiliferous sands and gravels occur, which I am inclined (provisionally) to assign to this period. They rest upon a bed containing erratic blocks of granite, and other rocks of northern origin; and I collected from them 13 species of mollusca. Only one species was peculiarly northern (_astarte borealis_) but all in the group have an arctic range of habitat. The elevation of these beds is about 357 feet above the sea.

At Fox Hall Field, New Lodge, Lilleshall (Shropshire) in a pit worked for sand, 463 feet above the sea, Mr. Woodward discovered 21 species of mollusca, three--viz., _Dentalium abyssorum_, _Natica affinis_, and _Astarte borealis_--being characteristically arctic and extinct in British waters.

_The Upper Boulder Clay_ is worked for bricks in many localities. It is distinguished from the Lower Boulder Clay, by having erractic blocks sparsely scattered through it. It is often very compact and tenacious. No fossils have as yet been found in it; unless indeed a clay derived from a drain in a street at Wolverhampton, in which I detected fragments of _Tellina balthica_, the spine of an _Echinus_, _Polymorphina lactea_, and _Polystomella crispa_, may be assigned to this division of the epoch.

The extraordinary _dispersion of erratic blocks_ over the surface of the ground remains to be noticed, and constitutes one of the most remarkable phenomena in local glacial geology. I distinguish the boulders resting on the surface of the ground from those embedded in the clay beds, although it is of course possible and probable that the clay has been largely denuded, and the boulders have thus been left exposed. Many of these erratic blocks may therefore belong to the Lower Boulder Clay; while others may have fallen from the icebergs which during the proved subsidence of the land must have floated over the “Midland” sea, and have been deposited in the Upper boulder clay, while it was in process of accumulation.

How far the dispersion of erratics over the Midlands may be referred to the ice sheet of some geologists, or to the icebergs of the Archipelago period in the history of Great Britain, must, at present, be regarded as an open question.

The Midland erratics have undoubtedly travelled from three distinct regions, viz., (1) from Wales, (2) the western part of the Lake district, and (3) Kirkcudbrightshire. Boulders from the more easterly part of the Lake district, such as the Snap granite boulders, so abundantly spread over Yorkshire, have not been found in this neighbourhood.

The peculiar distribution of the Midland erratics is noteworthy. Commencing at Bushbury Hill (a little to the north of Wolverhampton, on the table land facing towards the N.W.) the Lake rocks and the Scotch rocks--Criffell granites and Eskdale granites--are largely intermingled. Journeying westwards, a stream of boulders from Wales crosses the northern streams. On and around the Clent Hills (1023 feet) south west of Birmingham, Welsh felsites are the only boulders to be found. Birmingham itself being in the rear of the higher part of the table land on which it stands, is in a kind of protected district, so far as the northern stream of boulders is concerned, and the erratics in its immediate vicinity are chiefly Welsh felsites; a few fragments however of granite are occasionally found.

POST GLACIAL BEDS.--The most complete section of post glacial beds in this locality was obtained during excavations made at Shustoke, near Birmingham; when a bed of black peat, containing the remains of _Elephas antiquus_, _Cervus elephas_, and _Bos primigenius_, with hazel nuts and fragments of wood, was discovered 7ft. 6in. beneath the surface. The section was as follows:--

Soil 1ft. 0in.
Sandy marl 1ft. 8in.
Yellow clay (stiff) 3ft. 8in.
Blue clay (stiff) 1ft. 2in.
Black peat 1ft. 10in.
“Ballast” gravel and sand 3ft. 0in.
Sandstone and marl.

The fossils found in the peat have been placed in the Geological Museum of the Mason College.

PETROGRAPHY.

Notes on the Igneous and Metamorphic Rocks of the Birmingham District.

BY S. ALLPORT, F.G.S.

The space available for some account of the crystalline rocks being strictly limited, it would be useless to attempt anything more than a brief general description of the most important and interesting varieties. Fortunately there is abundance of material, for Birmingham, as a central point, affords unusual facilities for the study of this branch of petrology. Although the rocks here described appear to be scattered over a rather wide area, it will be found that every locality mentioned may not only be easily reached from one of the railway stations, but that a good series of specimens may be collected and the return journey made within the same day.

THE MALVERN HILLS.--It was clearly shown by Dr. Holl, in 1865, (Quar. Jour. Geol. Soc., Vol. xxi.) that the central portion or axis of this chain of hills consists of a great series of true crystalline schists, among which the prevailing types are hornblendic and micaceous gneiss, hornblende-schist, mica-schist, and a quartzo-felspathic rock, all of which are more or less distinctly foliated. There is, however, great variety in the relative proportions of the constituents; in many places either the hornblende or the mica are nearly, or even quite absent; the felspar and quartz then form the mass of the rock. In the quarries on the east side of the North Hill, beautiful examples of hornblende and felspar rock are abundant; they contain more or less quartz, with a little mica, and occasionally pass by insensible gradations from a well-marked gneissic structure into a coarsely crystalline mass in which foliation is no longer apparent. These latter are, however, exceptional cases, and there can, I think, be no doubt that we have here, in the Midlands, a considerable exposure of the oldest type of foliated crystalline schists, or as they are now frequently called Pre-Cambrian or Archæan rocks. The rocks of the North Hill are probably the oldest; they have been much disturbed, and are generally the most coarsely crystalline of the series. The more basic portions have suffered a considerable amount of alteration; the secondary constituents, chlorite and epidote, become abundant, and occasionally the hornblende has been completely replaced by the latter mineral; the rock then becomes an epidote schist of a pale-yellowish colour. In Swinyards Hill there is a micaceous gneiss with garnets, and in Raggedstone Hill there are interesting varieties of contorted mica-schist containing a large proportion of quartz. Dykes of intrusive rocks occur in nearly all the hills, but become far more numerous towards the north; they are very uniform in appearance, and are probably altered dolerites or diorites; very few specimens have been examined, and they belong to the former group. In the North Hill there are some masses of true diorite, but their relation to the hornblendic gneisses with which they are associated has not been clearly established.

MOUNT SORREL GRANITE.--The granite is of two varieties, red and grey, the difference being due to the fact, that in the red masses, the partially decomposed felspar has been coloured by ferric oxide. The rock is a hornblendic granite, the constituents being quartz, felspar, biotite, hornblende and titanite, with magnetite and a little apatite. The felspars are orthoclase and plagioclase; the former is much decomposed, while the plagioclase frequently remains clear, and exhibits well its twin striation. Biotite was originally abundant, but is very frequently replaced by a clear green substance, which is strongly dichroic, the two colours being grass-green, and clear yellow. The hornblende has been greatly decomposed; clear crystals are, however, not uncommon, and exhibit the usual optical characters of the mineral. The products of alteration are chlorite and epidote. The titanite appears in reddish-brown grains, but is not very abundant. In 1879, the writer discovered the junction of the granite with the sedimentary rocks, and proved that the former was intrusive (Geol. Mag. dec. ii., Vol. vi., p. 181). The junction occurs in Brazil Wood, where there is a small quarry, in which granite veins may be seen to penetrate the strata in various directions; a large mass of granite is also within a few yards. Some of the phenomena of _contact metamorphism_ may here be readily studied, the granite having converted the slate into a crystalline micaceous schist, quite similar in character to those produced under like conditions round the granite in Cornwall and elsewhere. Small garnets occur in the altered slates, and also in the granite close to the junction.

THE CHARNWOOD SYENITES.--The Syenites and other igneous rocks of Charnwood, have been described by Messrs. Hill and Bonney (Quar. Jour. Geol. Soc. Vol. xxxiv., p. 199.) The original constituents are felspar, hornblende, quartz, apatite, ilmenite, magnetite and titanite. The felspar is of two kinds, orthoclase and plagioclase, the former is very turbid and decomposed, while the plagioclase is clear, and retains its usual optical characters. The curious intercrystallization of quartz and felspar, known as _micropegmatite_, is common in the masses of syenite near Groby; the best examples, however, have been found by the writer in the Markfield rock, where it appears to form a ground-mass in which the larger crystals of orthoclase and plagioclase are enclosed. A small portion of the hornblende is still characteristic, but the greater portion appears in various stages of decomposition; the alteration products being chlorite and epidote. Titanite is by no means rare, and occurs in well formed twin crystals.

DIORITES OF ATHERSTONE AND NUNEATON.--A careful examination of many specimens collected by the writer from the various masses marked in the map 63 S.W. of the Geological Survey shows clearly that they are diorites, the characteristic constituents being hornblende and a triclinic felspar; these minerals, together with magnetite and apatite are invariably present, and in addition, a little orthoclase is seldom absent. The best specimen examined is from a quarry near Marston Jabet; it is a fine-grained rock, similar to a basalt in external appearance, but contains numerous crystals of hornblende and plagioclase which are generally quite unaltered. The clear brown crystals of hornblende are unusually well developed, and afford excellent opportunities of examining their crystallographic and optical characters. There are also present many grains of magnetite, and a few needles of apatite. The only product of alteration is a little calcite in the spaces between the crystals. Other specimens from the same quarry will perhaps give a better idea of the general character of the rocks of the district, and they possess a special interest, as they afford unusually good examples of successive stages of alteration. In one specimen the constituents are well preserved, the plagioclase is clear and exhibits its characteristic twin striation; the hornblende is, for the most part, unaltered, but is much fissured, and occasionally contains so many cavities that the crystals are little more than skeletons. A ground-mass in which the constituents were set has been highly altered, and now consists of a fine granular substance, partly serpentinous, with here and there a little calcite. A second example is quite similar in texture to the last; the felspar, still recognisable as triclinic, has been partially converted into a grey turbid substance. The hornblende occurs in various stages of alteration; some crystals are but slightly attacked, while others are to a considerable extent replaced by a pale green serpentinous substance. The alteration has followed the cleavage-lines and fractures, while the numerous cavities just mentioned are also filled by the same substance. In a single slice there may be seen almost every degree of change from a slight marginal erosion to a mere skeleton of the original. Of the latter, however, some little is always left; and whether the alteration be little or great, the crystalline forms are perfectly preserved. In a third specimen the alteration has proceeded still further, the whole of the hornblende crystals having been completely converted into pale green pseudomorphs; they were originally large and well developed, and their forms are still perfectly sharp and distinct. The felspar is here quite turbid and opaque, and the interstitial ground-mass is represented by calcite.

_Purley Park, near Atherstone._--The rock here contains, in addition to the usual brown hornblende, many crystals and grains of clear yellowish augite, and several pseudomorphs after olivine. The augite crystals exhibit the usual forms, some being twins. The pseudomorphs after olivine are quite similar to others observed in certain highly altered dolerites, they consist of calcite or calcite and viridite; they are numerous, and are generally larger than the crystals of augite or hornblende.

_Quarry close to Atherstone._--This rock also contains both augite and hornblende; and lastly, in the railway cutting at Chilvers Coton, several interesting varieties of diorite may be found.

THE RHYOLITES OF THE WREKIN NEAR WELLINGTON, SHROPSHIRE.--In the large quarry in Lawrence Hill, at the north end of the Wrekin, and at Lea Rock on the Shrewsbury Road, are to be found some of the most beautiful varieties of ancient volcanic glassy rocks hitherto discovered in Britain. The rocks in their present condition do not look like glass, owing to a process of devitrification, which they have evidently undergone. There can, however, be no room for doubt as to their original vitreous condition, for they exhibit, under the microscope, certain peculiar perlitic and spherulitic varieties of structure, associated with characteristic forms of microliths, which are found only in the pitchstone and obsidian varieties of volcanic glass. In the quarry in Lawrence Hill, thick beds of volcanic ashes and agglomerates are to be seen dipping towards the north at a high angle, and an examination of this hill and the Wrekin shows that they both consist of a series of stratified ashes alternating with several flows of rhyolite. One of the ash beds contains numerous spheroidal blocks of thoroughly characteristic varieties of glassy rocks; they have not yet been described, but it may here be stated that, in addition to many typical varieties of known rhyolites, there are also included among them some of the rarer glassy kinds described by Zirkel in his Petrography of the 40th Parallel. Beautiful examples of the spherulitic and perlitic varieties may be found at Lea Rock, and have been described by the writer in the Quar. Jour. Geol. Soc. Vol. xxxiii., p. 449. It has been shown by Dr. Callaway, that all these rhyolites are of Pre-Cambrian age; we have here, therefore, the clearest proof that, during very early geological periods volcanic action was of the same kind, and produced the same results, as in more recent times.

SOUTH STAFFORDSHIRE.--The igneous rocks of the South Staffordshire coalfield belong to the basic series, and are, without exception, dolerites or basalts, the latter being merely a fine grained variety of the former. They have been intruded among the coal measures and shales, and are frequently found in an excellent state of preservation. The original mineral constituents are crystals, or crystalline grains of triclinic felspar, augite, olivine, magnetite, ilmenite, and apatite. These minerals are very frequently quite unaltered, with the exception of the olivine, which is often partly converted into serpentine; this is the pale green substance seen along the cracks, and around the sides of the grains; generally, however, the decomposition has been continued until the formation of complete serpentinous pseudomorphs after olivine has been the result. The Hailstone Hill, near Rowley Church, is the best locality for varieties in texture and composition, as also for contemporaneous veins. In the large quarry there is a very coarsely crystalline variety containing large flat plates of ilmenite, and here also may be found some light-coloured veins in which orthoclase is the predominant felspar. The writer has also found in the same quarry vessicular and amygdaloidal varieties of the rock. It need scarcely be mentioned that, minute details of structure and composition can only be studied in thin slices under the microscope. Rocks of similar character to the above occur in the following localities:--Pouk Hill, near Walsall; Titterston Clee Hill (sheet 55, N.W.), Knowl Hill, near Kinlet (sheet 55, N.E.), and Swinnerton Park, eight miles N.E. of Stafford. For fuller descriptions of these rocks, see Allport, Quart. Jour. Geol. Soc., Vol. xxx., p. 529.

Mining Statistics of the South Staffordshire Coalfield.

By PROFESSOR W. E. BENTON, A.R.S.M., F.G.S.

MINERALS WORKED.--The principal minerals worked within the limits of the South Staffordshire Coalfield are coal, ironstone, and fireclay from the Carboniferous formations, and limestone from the underlying Silurian rocks.

THE COAL is bituminous, non-caking, and much of it contains a high percentage of water and oxygen. Some of the seams produce excellent house-coal, others manufacturing and steam coal.

THE IRONSTONE is an argillaceous carbonate of iron, occurring as nodules in the roofs of the coal seams, or as thin beds within them.

THE FIRECLAY, particularly in the neighbourhood of Stourbridge, has a high refractory power.

QUANTITY OF MINERALS RAISED.--The annual produce of minerals during the last twenty years has remained without any considerable alteration. The Government Mineral Statistics for 1885 are not yet published, but in 1884 the quantities of minerals raised in South Staffordshire Coalfield were:--Coal, 9,688,047 tons; ironstone, 116,951 tons; fireclay, 205,320 tons; limestone (no statistics given). The respective values of these minerals at the mines were estimated at:--coal, £2,785,313; ironstone, £62,974; fireclay, £42,781.

The total number of persons employed at the mines during the same period was 23,782.

DESTINATION OF THE MINERALS.--The greater part of the coal is consumed in the district for house purposes, for steam raising, for the manufacture of bricks, pottery, glass, salt, &c. The remainder is carried out of the coalfield into the south and south-western districts of England by the London and North Western, Great Western, and Midland Railway Companies, for house and steam purposes. The gas coals and coke used in the Birmingham district are not the product of the South Staffordshire Coalmeasures, but are brought principally from North Staffordshire, Derbyshire, and Yorkshire.

The whole of the ironstone raised is smelted within the district, in addition to large quantities of iron ore (hydrated oxide) brought from the Northampton and North Staffordshire mines. The manufacture of pig iron has within the last decade decreased to such an extent that less than one-third of the existing furnaces of South Staffordshire are at present in blast.

The quantity of pig produced in the coalfield in 1884, was 356,873 tons; in the reduction of which 810,936 tons of coal (including coal converted into coke) were used, or about 45½ cwts. of coal to one ton of pig iron. The whole of the pig iron is retained in the district. The finished iron trade retains its importance, more than one-third of the existing puddling furnaces and rolling mills in Great Britain occur within the limits of the South Staffordshire Coalfield.

The finished iron is produced from local pig iron, and from pig iron brought from Derbyshire, Yorkshire, and North Staffordshire. The steel production in South Staffordshire is gradually increasing. The processes adopted are those known as the Bessemer, Gilchrist and open hearth.

The South Staffordshire fireclay is worked principally in the neighbourhood of Brettle Lane and Stourbridge, and is employed in the production of firebricks, gas retorts, pottery, etc. Much also is conveyed into other districts in a raw condition for pottery purposes.

The Silurian limestone is worked partly in open work in the neighbourhood of Dudley, and partly by ordinary underground mining operations at the Wren’s Nest, Sedgley, and at Walsall.

CHARACTERISTIC FEATURES OF THE MINING OF SOUTH STAFFORDSHIRE.--There are several noteworthy features in the mining of South Staffordshire. A stranger is especially impressed with the large number of separate collieries in working (about 600) in proportion to the quantity of mineral raised. This peculiarity is due essentially to the insignificant depth at which the minerals occur below the surface (indeed, at Foxyard, near Tipton, coal has long been quarried in an outcrop of the 10 yard seam). This “Shallow Mining” has passed its meridian. The future mining of the district is forecast by those remarkable operations on the “red ground,” forming the eastern side of the coalfield. The most recently opened seam in the red ground is at Hampstead, three miles north of Birmingham, where coal mining is carried on below the Permian and Triassic, at a depth of more than 600 yards.

Another feature of the mining is the unique South Staffordshire mode of getting coal in the ten yard seam, called “Square Work.” This method has met with much condemnation from strangers, but after trial of other methods, it still dominates. A further and most strongly marked peculiarity of the South Staffordshire area is the tendency of some of the Black Country coals to spontaneous ignition. Much coal has thus in times past been sacrificed; but a better acquaintance with the causes of this phenomenon has led to measures which have reduced and which must still further reduce this sacrifice of wealth.

MINES’ DRAINAGE.--In the Tipton and Old Hill districts many of the coal mines are water-logged. In 1873 a Parliamentary Commission however was appointed to drain this area, and was empowered to levy rates to defray the drainage expenses. The drainage area under the direction of this Mines’ Drainage Commission is 50 square miles. The principal pumping stations are the Moat, the Stoneheath Station, and the Bradley Station. The Bradley pumping engines (a quarter of a mile from Moxley on the Great Western Railway) are of the compound type having 52-inch and 90-inch steam cylinders. These engines, with a 10 feet stroke, and six strokes per minute, work two 27-inch plunger pumps, and raise from a depth of 126 yards more than 4,000,000 gallons of water per 24 hours. As a result of this gallant effort to recover these water-logged minerals, the number of pumping stations has been gradually reduced in the Tipton district from 77 in 1873 to 10 in 1885, and the quantity of water from 23,000,000 to 10,000,000 of gallons daily. To meet the inevitable expenses, a rate of 9d. per statute ton is levied on all coals, slack, and ironstone; 3d. per statute ton on all fireclay and limestone, and 1d. per statute ton for surface drainage on all minerals, raised in the Tipton district. In the Old Hill district a much lower mines’-drainage rate is collected.

LITERATURE.

BOOKS, PAPERS AND MAPS BEARING UPON THE GEOLOGY OF THE BIRMINGHAM DISTRICT.

_Fundamental Gneissic and Volcanic Rocks._

Phillips, Prof., “On the Geology of the Malvern and Abberley Hills.” Memoirs of the Geological Survey, Vol. ii., 1848.

Holl, Dr. H. B., “On the Geology and Structure of the Malvern Hills.” Quart. Journal Geol. Soc., Vol. xxi., p. 72.

Callaway, Dr. C., “On a second Pre-Cambrian Group in the Malvern Hills.” Q. J. G. S., 1880, Nov. 1880.

Prof. T. G. Bonney, LL.D., and Rev. W. Hill, M.A. “On the Pre-Carboniferous Rocks of Charnwood Forest.” Q. J. G. S., 1877, p. 754, &c.

Allport. S., “Ancient Devitrified Pitchstones and Perlites from Lower Silurian District of Shropshire.” Quart. Journ. Geol. Soc., 1877, p. 439.

Callaway, Dr. Chas., “The Pre-Cambrian Rocks of Shropshire.” Quart. Jour. Geol. Soc. 1879, p. 643; ibid, 1882, p. 120.

_Cambrian Formations._

Prof. Phillips, “Geology of Malvern and Abberley Hills.” See above.

Holl, Dr. H., “Geology and Structure of the Malvern Hills.” See above.

Callaway, Dr. C., “On a new Area of Upper Cambrian Rocks in South Shropshire.” Q. J. G. S., 1877. p. 652.

T. S. Houghton., M.A., F.G.S. “Note on the Age of the Quartzite of the Lickey.” Proceedings Birmingham Philosophical Society, 1881-2, p. 206.

Prof. C. Lapworth, “On the Discovery of Cambrian Rocks in the neighbourhood of Birmingham;” ibid, p. 234. See also Geological Magazine, 1882, p. 563; ibid, July, 1886.

W. Jerome Harrison, F.G.S., “On the Pre-Carboniferous Floor of the Midlands.” Midland Naturalist, 1855, p. 38.

_Silurian Rocks._

Sir R. J. Murchison, “Silurian System,” p. 408, and 480.

J. Beete Jukes, F.R.S. “The Geology of the South Staffordshire Coalfield.” 2nd Ed., p. 145 et seq.

_Carboniferous Rocks._

Sir R. J. Murchison, “Silurian System,” p. 463.

J. Beete Jukes, F.R.S. “The Geology of the South Staffordshire Coalfields.” 2nd Edition; see also Jukes, “On the Geological Structure of South Staffordshire Coalfield.” Birmingham and Midland and Hardware District, 1866, p. 1.

Howell, H., F.G.S., &c. “The Geology of the Warwickshire Coalfield.” Mem. Geol. Survey, 1859.

_Permian and Triassic Rocks._

Hull, Prof., M.A., F.R.S. “Triassic and Permian Rocks of the Midlands.” Memoirs Geol. Survey.

Sir And. Ramsay, F.R.S., “On the occurrence of Angular boulders in the Permian Breccia of Shropshire and Worcestershire.” Quart. Jour. Geol. Soc., 1855, p. 185.

Rev. P. B. Brodie, “Upper Keuper Sandstone of Warwickshire.” Journ. Geol. Soc., Vol. xii., p. 374.

Prof. L. C. Miall, “Labyrinthodonts from the Keuper Sandstone, in the Warwick Museum.” Journ. Geol. Soc., Vol. xxx., p. 417.

W. J. Harrison, “On the Quartzite Pebbles contained in the Triassic Strata of England, and on their Derivation from an Ancient Land Barrier in Central England.” Proc. Birmingham Phil. Soc., Vol. iii., p. 157.

Prof. T. G. Bonney, “On the Pebbles in the Bunter Beds.” Geol. Mag. for 1880, p. 404.

Thos. Davidson, “Brachiopoda of the Budleigh-Salterton Pebble Bed.” Q. J. G. S., Vol. xxvi., p. 70. 1870.

_Liassic and Rhætic Rocks._

Rev. P. B. Brodie, “Lias Outliers at Knowle and Wooton Wawen.” Journ. Geol. Soc., Vol. xxi., p. 159. 1865.

R. F. Tomes, “Corals of the Lias.” Journ. Geol. Soc., Vol. xxxiv., p. 179. 1878.

W. J. Harrison, “The Rhætic Section at Dunhampstead, near Droitwich.” Proc. Dudley Geol. Soc., Vol. iii., p. 115.

_Glacial and Post Tertiary Formations._

“The Direction and Limits of Dispersion, etc., of the Erratic Blocks of the West of England and East of Wales,” by D. Mackintosh, F.G.S. Quarterly Journal Geological Society, Vol. xxxv., p. 425.

“The Correlation of the Drift Deposits of the N.W. of England, with those of the Midland and Eastern Counties,” by D. Mackintosh, ibid, Vol. xxxvi., p. 178.

“Post Tertiary Beds of the Midland District,” by H. W. Crosskey, LL.D., and C. J. Woodward. Proceedings of the Birmingham Natural History Society, 1873, p. 43.

“On a Section of Glacial Drift, recently exposed in Icknield Street, Birmingham.” By H. W. Crosskey. Proceedings of Philosophical Society of Birmingham. Vol. iii. p. 209.

“The Grooved Blocks and Boulder Clays of Rowley Hill.” By H. W. Crosskey. Ibid, Vol. iii., p. 459; and Vol. iv., p. 69.

“Reports of the Committee of the British Association on the Distribution, etc., of Erratic Blocks; drawn up by H. W. Crosskey.” 1873-1885.

“The Geological Section along the West Suburban Railway from Birmingham to King’s Norton.” By F. W. Martin. Proceedings of Philosophical Society of Birmingham, Vol. iv., p. 257.

_Petrography._

S. Allport, F.G.S., “Diorites of E. Warwickshire Coalfield.” Q. J. G. S.

S. Allport, F.G.S., “Carboniferous Dolerites.” Q. J. G. S., xxx., p. 529.

S. Allport, F.G.S., “Vitreous Rocks of the Wrekin,” ibid, xxxiii., 449.

See also the Papers by Professor Phillips, Dr. Holl, and Dr. C. Callaway, Rev. Professor Bonney and Rev. T. Hill, cited above.

T. H. Waller, B.Sc., “Observations on the Structure of the Rowley Rag.” Midland Naturalist, 1885, p. 261.

_Maps of Birmingham District._

Published by H.M. Geological Survey.

62--S.E. Birmingham.

62--N.W. Penkridge.

63--N.W. Market Bosworth.

53--N.W. Coventry.

54--N.W. Droitwich.

62--S.W. Dudley.

62--N.E. Lichfield.

63--S.W. Nuneaton.

54--N.E. Henley-in-Arden.

* * * * *

NOTE.--The main outlines and colors of the formations &c., in the accompanying Geological Sketch-Map correspond with those of the above mentioned one inch Survey Maps of the District.