Chapter XVIII

FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF.

Cambrian Conglomerate of Islay and North-west of
Scotland.—Ice-action in Ayrshire and Wigtownshire
during Silurian Period.—Silurian Limestones in Arctic
Legions.—Professor Ramsay on Ice-action during Old
Red Sandstone Period.—Warm Climate in Arctic Regions
during Old Red Sandstone Period.—Professor Geikie and
Mr. James Geikie on a Glacial Conglomerate of Lower
Carboniferous Age.—Professor Haughton and Professor Dawson
on Evidence of Ice-action during Coal Period.—Mr. W. T.
Blanford on Glaciation in India during Carboniferous
Period.—Carboniferous Formations of Arctic Regions.—Professor
Ramsay on Permian Glaciers.—Permian Conglomerate in
Arran.—Professor Hull on Boulder Clay of Permian Age.—Permian
Boulder Clay of Natal.—Oolitic Boulder Conglomerate in
Sutherlandshire.—-Warm Climate in North Greenland during
Oolitic Period.—Mr. Godwin-Austen on Ice-action during
Cretaceous Period.—Glacial Conglomerates of Eocene Age in the
Alps.—M. Gastaldi on the Ice-transported Limestone Blocks of
the Superga.—Professor Heer on the Climate of North Greenland
during Miocene Period 292

CHAPTER XIX.

GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH.

Geological Time measurable from Astronomical Data.—M. Leverrier’s
Formulæ.—Tables of Eccentricity for 3,000,000 Years in the
Past and 1,000,000 Years in the Future.—How the Tables have
been computed.—Why the Glacial Epoch is more recent than had
been supposed.—Figures convey a very inadequate Conception
of immense Duration.—Mode of representing a Million of
Years.—Probable Date of the Glacial Epoch 311

CHAPTER XX.

GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL
DENUDATION.

Rate of Subaërial Denudation a Measure of Time.—Rate determined
from Sediment of the Mississippi.—Amount of Sediment carried
down by the Mississippi; by the Ganges.—Professor Geikie on
Modern Denudation.—Professor Geikie on the Amount of Sediment
conveyed by European Rivers.—Rate at which the Surface of
the Globe is being denuded.—Alfred Tylor on the Sediment
of the Mississippi.—The Law which determines the Rate of
Denudation.—The Globe becoming less oblate.—Carrying Power
of our River Systems the true Measure of Denudation.—Marine
Denudation, trifling in comparison to Subaërial.—Previous
Methods of measuring Geological Time.—Circumstances which
show the recent Date of the Glacial Epoch.—Professor Ramsay
on Geological Time 329

CHAPTER XXI.

THE PROBABLE AGE AND ORIGIN OF THE SUN.

Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric
Theory.—Helmholtz’s Condensation Theory.—Confusion of
Ideas.—Gravitation not the chief Source of the Sun’s
Heat.—Original Heat.—Source of Original Heat.—Original Heat
derived from Motion in Space.—Conclusion as to Date of
Glacial Epoch.—False Analogy.—Probable Date of Eocene and
Miocene Periods 346

CHAPTER XXII.

A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY
ROCKS OF THE GLOBE.

Prevailing Methods defective.—Maximum Thickness of British
Rocks.—Three Elements in the Question.—Professor Huxley
on the Rate of Deposition.—Thickness of Sedimentary Rocks
enormously over-estimated.—Observed Thickness no Measure of
mean Thickness.—Deposition of Sediment principally along
Sea-margin.—Mistaken Inference regarding the Absence of a
Formation.—Immense Antiquity of existing Oceans 360

CHAPTER XXIII.

THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE
OF THE LAND DURING THE GLACIAL EPOCH.

Displacement of the Earth’s Centre of Gravity by Polar
Ice-cap.—Simple Method of estimating Amount of
Displacement.—Note by Sir W. Thomson on foregoing
Method.—Difference between Continental Ice and
a Glacier.—Probable Thickness of the Antarctic
Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The
Icebergs of the Southern Ocean.—Inadequate Conceptions
regarding the Magnitude of Continental Ice 368

CHAPTER XXIV.

THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE
LAND DURING THE GLACIAL EPOCH.—(_Continued._)

Extent of Submergence from Displacement of Earth’s Centre
of Gravity.—Circumstances which show that the Glacial
Submergence resulted from Displacement of the Earth’s
Centre of Gravity.—Agreement between Theory and Observed
Facts.—Sir Charles Lyell on submerged Areas during
Tertiary Period.—Oscillations of Sea-Level in Relation to
Distribution.—Extent of Submergence on the Hypothesis that
the Earth is fluid in the Interior 387

CHAPTER XXV.

THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE
AND ON THE LEVEL OF THE SEA.

The direct Effect of Change of Obliquity on Climate.—Mr.
Stockwell on the maximum Change of Obliquity.—How Obliquity
affects the Distribution of Heat over the Globe.—Increase of
Obliquity diminishes the Heat at the Equator and increases
that at the Poles.—Influence of Change of Obliquity on the
Level of the Sea.—When the Obliquity was last at its superior
Limit.—Probable Date of the 25-foot raised Beach.—Probable
Extent of Rise of Sea-level resulting from Increase of
Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s
Theories.—Sir Charles Lyell on Influence of Obliquity 398

CHAPTER XXVI.

COAL AN INTER-GLACIAL FORMATION.

Climate of Coal Period Inter-glacial in Character.—Coal Plants
indicate an Equable, not a Tropical Climate.—Conditions
necessary for Preservation of Coal Plants.—Oscillations
of Sea-level necessarily implied.—Why our Coal-fields
contain more than One Coal-seam.—Time required to form a
Bed of Coal.—Why Coal Strata contain so little evidence of
Ice-action.—Land Flat during Coal Period.—Leading Idea of the
Theory.—Carboniferous Limestones 420

CHAPTER XXVII.

PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS
TO THE BOULDER CLAY OF CAITHNESS.

Character of Caithness Boulder Clay.—Theories of the Origin
of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W.
Peach’s Theory.—The proposed Theory.—Thickness of Scottish
Ice-sheet.—Pentlands striated on their Summits.—Scandinavian
Ice-sheet.—North Sea filled with Land-ice.—Great Baltic
Glacier.—Jutland and Denmark crossed by Ice.—Sir R.
Murchison’s Observations.—Orkney, Shetland, and Faroe Islands
striated across.—Loess accounted for.—Professor Geikie’s
Suggestion.—Professor Geikie and B. N. Peach’s Observations
on East Coast of Caithness.—Evidence from Chalk Flints and
Oolitic Fossils in Boulder Clay 435

CHAPTER XXVIII.

NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG
BLOCKS.

Transport of Blocks; Theories of.—Evidence of Continental
Ice.—Pennine Range probably striated on Summit.—Glacial
Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold
Hills.—England probably crossed by Land-ice.—Mr. Jack’s
Suggestion.—Shedding of Ice North and South.—South of England
Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are
so rare in South of England.—Form of Contortion produced by
Land-ice 456

CHAPTER XXIX.

EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD
IN BRITAIN.

Remarks on the Drift Deposits.—Examination of Drift
by Borings.—Buried River Channel from Kilsyth to
Grangemouth.—Channels not excavated by Sea nor by
Ice.—Section of buried Channel at Grangemouth.—Mr. Milne
Home’s Theory.—German Ocean dry Land.—Buried River Channel
from Kilsyth to the Clyde.—Journal of Borings.—Marine
Origin of the Drift Deposits.—Evidence of Inter-glacial
Periods.—Oscillations of Sea-Level.—Other buried River
Channels 466

CHAPTER XXX.

THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF
GLACIER-MOTION.

Why the Question of Glacier-motion has been found to be so
difficult.—The Regelation Theory.—It accounts for the
Continuity of a Glacier, but not for its Motion.—Gravitation
proved by Canon Moseley insufficient to shear the Ice
of a Glacier.—Mr. Matthew’s Experiment.—No Parallel
between the bending of an Ice Plank and the shearing
of a Glacier.—Mr. Ball’s Objection to Canon Moseley’s
Experiment.—Canon Moseley’s Method of determining the Unit
of Shear.—Defect of Method.—Motion of a Glacier in some
Way dependent on Heat.—Canon Moseley’s Theory.—Objections
to his Theory.—Professor James Thomson’s Theory.—This
Theory fails to explain Glacier-motion.—De Saussure and
Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation”
Theory.—Important Element in the Theory 495

CHAPTER XXXI.

THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE MOLECULAR
THEORY.

Present State of the Question.—Heat necessary to the Motion of
a Glacier.—Ice does not shear in the Solid State.—Motion
of a Glacier _molecular_.—How Heat is transmitted through
Ice.—Momentary Loss of Shearing Force.—The _Rationale_
of Regelation.—The Origin of “Crevasses.”—Effects of
Tension.—Modification of Theory.—Fluid Molecules crystallize
in Interstices.—Expansive Force of crystallizing Molecules
a Cause of Motion.—Internal molecular Pressure the chief
Moving Power.—How Ice can excavate a Rock Basin.—How Ice can
ascend a Slope.—How deep River Valleys are striated across.—A
remarkable Example in the Valley of the Tay.—How Boulders can
he carried from a lower to a higher Level 514