NEWER
PLIOCENE.
The shells almost all of living species. British
Bridlington beds, marine Arctic fauna (p. 189).
Glacial boulder formation of Norfolk cliffs (p. 190).
Forest-bed of Norfolk cliffs, with bones of _Elephas meridionalis,_
etc. (p. 191).
Chillesford and Aldeby beds, with marine shells, chiefly Arctic (p.
192).
Norwich crag (p. 193).
Foreign
Eastern base of Mount Etna, with marine shells (p. 204).
Sicilian calcareous and tufaceous strata (p. 205, 206).
Lacustrine strata of Upper Val d’Arno (p. 207).
Madeira leaf-bed and land-shells (p. 532). 4.
OLDER
PLIOCENE.
Extinct species of
shells forming a
large minority. British
Red crag of Suffolk, marine shells, some of northern forms (p. 194,
195).
White or coralline crag of Suffolk (p. 197).
Foreign
Antwerp crag (p. 204).
Subapennine marls and sands (p. 208).
EXAMPLES
MIOCENE 5.
UPPER
MIOCENE.
Majority of the
shells extinct. British
Wanting.
Foreign
Faluns of Touraine (p. 211).
Faluns, proper, of Bordeaux (p. 214).
Fresh-water strata of Gers (p. 215).
Swiss Oeningen beds, rich in plants and insects (pp. 215-23).
Marine Molasse, Switzerland (p. 223).
Bolderberg beds of Belgium (p. 224).
Vienna basin (p. 224).
Beds of the Superga, near Turin (p. 226).
Deposit at Pikermé, near Athens (p. 226).
Strata of the Siwâlik hills, India (p. 226).
Marine strata of the Atlantic border in the United States (p. 227).
Volcanic tuff and limestone of Madeira, the Canaries, and the Azores
(). 6.
LOWER
MIOCENE.
Nearly all the
shells extinct. British
Hempstead beds, marine and fresh-water strata (p. 244).
Lignites and clays of Bovey Tracey (p. 245).
Isle of Mull leaf-bed, volcanic tuff (p. 247).
Foreign
Calcaire de la Beauce, etc. (p. 230).
Grès de Fontainebleau (p. 230).
Lacustrine strata of the Limagne d’Auvergne, and the Cantal (p. 233).
Mayence basin (p. 242).
Radaboj beds of Croatia (p. 242).
Brown coal of Germany (p. 244).
Lower Molasse of Switzerland, fresh-water and brackish (p. 235-9).
Rupelmonde, Kleynspawen, and Tongrian beds of Belgium (p. 241, 242).
Nebraska beds, United States (p. 248).
Lower Miocene beds of Italy (p. 244).
Miocene flora of North Greenland (p. 239). EOCENE 7.
UPPER
EOCENE. British
Bembridge fluvio-marine strata (p. 252).
Osborne or St. Helen’s series (p. 255).
Headon series, with marine and fresh-water shells (p. 255).
Barton sands and clays (p. 258).
Foreign
Gypsum of Montmartre, fresh-water with _Palæotherium_ (p. 270).
Calcaire silicieux, or Travertin inférieur (p. 273),
Grès de Beauchamp, or Sables moyens (p. 273). 8.
MIDDLE
EOCENE. British
Bracklesham beds and Bagshot sands (p. 259).
White clays of Alum Bay and Bournemouth (p. 262).
Foreign
Calcaire grossier, miliolitic limestone (p. 274).
Soissonnais sands, or Lits coquilliers, with _Nummulites planulata_ (p.
275).
Claiborne beds of the United States, with _Orbitoides_ and _Zeuglodon_
(p. 279).
Nummulitic formation of Europe, Asia, etc. (p. 277). 9.
LOWER
EOCENE. British
London clay proper (p. 263).
Woolwich and Reading series, fluvio-marine (p. 267).
Thanet sands (p. 269).
Foreign
Argile de Londres, near Dunkirk (p. 252).
Argile plastique (p. 276).
Sables de Bracheux (p. 276).
SECONDARY OR MESOZOIC.
CRETACEOUS 10.
UPPER
CRETACEOUS. British
Upper white chalk, with flints (p. 290).
Lower white chalk, without flints (p. 298).
Chalk marl (p. 298).
Chloritic series (or Upper Greensand), fire-stone of Surrey (p. 298).
Gault (p. 300).
Blackdown beds (p. 301).
EXAMPLES
CRETACEOUS 10.
UPPER
CRETACEOUS. Foreign
Maetricht beds and Faxoe chalk (p. 233).
Pisolitic limestone of France (p. 285).
White chalk of France, Sweden, and Russia (p. 286, 287).
Planer-kalk of Saxony (p. 293).
Sands and clays of Aix-la-Chapelle (p. 302).
Hippurite limestone of South of France (p. 305).
New Jersey, U.S., sands and marls (p. 307). 11.
LOWER
CRETACEOUS or
NEOCOMIAN. British
Sands of Folkestone, Sandgate, and Hythe (p. 308).
Atherfield clay, with _Perna mulleti_ (p. 309).
Punfield marine beds, with _Vicarya lujana_ (p. 318).
Speeton clay of Flamborough Head and Tealby (p. 311).
Weald clay of Surrey, Kent, and Sussex, fresh-water, with _Cypris_ (p.
313-5).
Hastings sands (p. 316-8).
Foreign
Neocomian of Neufchatel, and Hils conglomerate of North Germany (p.
312).
Wealden beds of Hanover (p. 319). OOLITE 12.
UPPER OOLITE. British
Upper Purbeck beds, fresh-water (p. 323).
Middle Purbeck, with numerous marsupial quadrupeds, etc. (p. 324).
Lower Purbeck, fresh-water, with intercalated dirt-bed (p. 330).
Portland stone and sand. (p. 334).
Kimmeridge clay (p. 335).
Foreign
Marnes à gryphées virgules of Argonne (p. 336).
Lithographic-stone of Solenhofen, with _Archæopteryx_ (p. 337). 13.
MIDDLE OOLITE. British
Coral rag of Berkshire, Wilts, and Yorkshire (p. 339).
Oxford clay, with belemnites and Ammonite (p. 340).
Kelloway rock of Wilts and Yorkshire (p. 341).
Foreign
Nerinæan limestone of the Jura (p. 339). 14.
LOWER OOLITE. British
Cornbrash and forest marble (p. 341).
Great or Bath oolite of Bradford (p. 342).
Stonesfield slate, with marsupials and _Araucaria_ (p. 345).
Fuller’s earth of Bath (p. 348).
Inferior oolite (p. 349). LIAS 15.
LIAS. Upper Lias, argillaceous, with _Ammonites striatulus_ (p.
353).
Shale and limestone, with _Ammonites bifrons_ (p. 353).
Middle Lias or Marlstone series, with zones containing characteristic
Ammonites (p. 353).
Lower Lias, also with zones characterised by peculiar Ammonites (p.
356). TRIAS 16.
UPPER TRIAS. British
Rhætic, Penarth or _Avicula contorta_ beds (beds of passage) (p. 366).
Keuper or Upper New Red sandstone, etc. (p. 369).
Red shales of Cheshire and Lancashire, with rock-salt (p. 371).
Dolomite conglomerate of Bristol (p. 373).
Foreign
Keuper beds of Germany (p. 375).
St. Cassian or Hallstadt beds, with rich marine fauna (p. 376).
Coal-field of Richmond, Virginia (p. 382).
Chatham coal-field, North Carolina (p. 383). 17.
MIDDLE TRIAS. British
Wanting.
Foreign
Muschelkalk of Germany (p. 378). 18.
LOWER TRIAS. British
Bunter or Lower New Red sandstone of Lancashire and Cheshire (p. 372).
Foreign
Bunter-sandstein of Germany (p. 380).
Red sandstone of Connecticut Valley, with footprints of birds and
reptiles (p. 381).
PRIMARY OR PALÆOZOIC
EXAMPLES
PERMIAN 19.
PERMIAN. British
Upper Permian of St. Bees’ Head, Cumberland (p. 386).
Middle Permian, magnesian limestone, and marl-slate of Durham and
Yorkshire, with _Protosaurus_ (p. 387).
Lower Permian sandstones and breccias of Penrith and Dumfriesshire,
intercalated (p. 390).
Foreign
Dark-coloured shales of Thuringia (p. 392).
Zechstein or Dolomitic limestone (p. 392).
Mergel-schiefer or Kupfer-schiefer (p. 392).
Rothliegendes of Thuringia, with _Psaronius_ (p. 392).
Magnesian limestones, etc., of Russia (p. 393). CARBONIFEROUS 20.
UPPER CARBONIFEROUS. British
Coal-measures of South Wales, with underclays inclosing _Stigmaria_ (p.
397).
Coal-measures of north and central England (p. 395).
Millstone grit (p. 395).
Yoredale series of Yorkshire (p. 395).
Coal-field of Kilkenny with _Labyrinthodont_ (p. 407).
Foreign
Coal-field of Saarbruck, with _Archegosaurus_ (p. 406).
Carboniferous strata of South Joggins, Nova Scotia (p. 409).
Pennsylvania coal-field (p. 403). 21.
LOWER CARBONIFEROUS. British
Mountain limestone of Wales and South of England (p. 430).
Same in Ireland (p. 437437).
Carboniferous limestone of Scotland alternating with coal-bearing
sandstones (p. 396).
Erect trees in volcanic ash in the Island of Arran (p. 546).
Foreign
Mountain limestone of Belgium (p. 436). DEVONIAN or
OLD RED SANDSTONE 22.
UPPER
DEVONIAN. British
Yellow sandstone of Dura Den, with _Holoptychius_, etc. (p. 440); and
of Ireland with _Anodon Jukesii_ (p. 441).
Sandstones of Forfarshire and Perthshire, with _Holoptychius_, etc. (p.
442).
Pilton group of North Devon (p. 449).
Petherwyn group of Cornwall, with _Clymenia_ and _Cypridina_ (p. 451).
Foreign
Clymenien-kalk and Cypridinen-schiefer of Germany (p. 450) 23.
MIDDLE
DEVONIAN. British
Bituminous schists of Gamrie, Caithness, etc., with numerous fish (p.
443).
Ilfracombe beds with peculiar trilobites and corals (p. 450).
Limestones of Torquay, with broad-winged Spirifers (p. 451).
Foreign
Eifel limestone, with underlying schists containing _Calceola_ (p.
453).
Devonian strata of Russia (p. 454). 24.
LOWER
DEVONIAN. British
Arbroath paving-stones, with _Cephalaspis_ and _Pterygotus_ (p. 446).
Lower sandstones of Forfarshire, with _Pterygotus_ (p. 446).
Sandstones and slates of the Foreland and Linton (p. 454).
Foreign
Oriskany sandstone of Western Canada and New York (p. 456).
Sandstones of Gaspe, with _Cephalaspis_ (p. 455 ).
EXAMPLES
SILURIAN 25.
UPPER SILURIAN British
Upper Ludlow formation, Downton sandstone, with bone-bed (p. 459).
Lower Ludlow formation, with oldest known fish remains (p. 461).
Wenlock limestone and shale (p. 465).
Woolhope limestone and grit (p. 467).
Tarannon shales (p. 468).
_Beds of passage between Upper and Lower Silurian:_
Upper Llandovery, or May-hill sandstone, with _Pentamerus oblongus_,
etc. (p. 468).
Lower Llandovery slates (p. 469).
Foreign
Niagara limestone, with _Calymene, Homalonotus_, etc. (p. 479).
Clinton group of America, with _Pentamerus oblongus_, etc. (p. 479).
Silurian strata of Russia, with _Pentamerus_ (p. 477). 26.
LOWER SILURIAN. British
Bala and Caradoc beds (p. 470).
Llandeilo flags (p. 473).
Arenig or Stiper-stones group (Lower Llandeilo of Murchison) (p. 475).
Foreign
Ungulite or Obolus grit of Russia (p. 477).
Trenton limestone, and other Lower Silurian groups of North America (p.
479).
Lower Silurian of Sweden (p. 477). CAMBRIAN 27.
UPPER CAMBRIAN. British
Tremadoc slates (p. 483).
Lingula flags, with _Lingula Davisii_ (p. 484).
Foreign
“Primordial” zone of Bohemia in part, with trilobites of the genera
_Paradoxides_, etc. (p. 487).
Alum schists of Sweden and Norway (p. 489).
Potsdam sandstone, with _Dikelocephalus_ and _Obolella_ (p. 489). 28.
LOWER CAMBRIAN. British
Menevian beds of Wales, with _Paradoxides Davidis_, etc. (p. 484).
Longmynd group, comprising the Harlech grits and Llanberis slates (p.
485).
Foreign
Lower portion of Barrande’s “Primordial” zone in Bohemia (p. 486).
Fucoid sandstones of Sweden (p. 489).
Huronian series of Canada? (p. 490). LAURENTIAN 29.
UPPER LAURENTIAN. British
Fundamental gneiss of the Hebrides? (p. 493).
Hypersthene rocks of Skye? (p. 491).
Foreign
Labradorite series north of the river St. Lawrence in Canada (p. 491).
Adirondack mountains of New York (p. 491). 30.
LOWER LAURENTIAN. British
Wanting?
Foreign
Beds of gneiss and quartzite, with interstratified limestones, in one
of which, 1000 feet thick, occurs a foraminifer, _Eozoon Canadense_,
the oldest known fossil (p. 491).
CHAPTER IX.
CLASSIFICATION OF TERTIARY FORMATIONS.
Order of Succession of Sedimentary Formations. — Frequent
Unconformability of Strata. — Imperfection of the Record. —
Defectiveness of the Monuments greater in Proportion to their
Antiquity. — Reasons for studying the newer Groups first. —
Nomenclature of Formations. — Detached Tertiary Formations scattered
over Europe. — Value of the Shell-bearing Mollusca in Classification. —
Classification of Tertiary Strata. — Eocene, Miocene, and Pliocene
Terms explained.
By reference to the tables given at the end of the last chapter the
reader will see that when the fossiliferous rocks are arranged
chronologically, we have first to consider the Post-tertiary and then
the Tertiary or Cainozoic formations, and afterwards to pass on to
those of older date.
Fig. 86: Order of Superposition of Deposits
Order of Superposition.—The diagram (Fig. 86) will show the order of
superposition of these deposits, assuming them all to be visible in one
continuous section. In nature, as before hinted (p. 107), we have never
an opportunity of seeing the whole of them so displayed in a single
region; first, because sedimentary deposition is confined, during any
one geological period, to limited areas; and secondly, because strata,
after they have been formed, are liable to be utterly annihilated over
wide areas by denudation. But wherever certain members of the series
are present, they overlie one another in the order indicated in the
diagram, though not always in the exact manner there represented,
because some of them repose occasionally in unconformable
stratification on others. This mode of superposition has been already
explained (p. 94, p. 111), where I pointed out that the discordance
which implies a considerable lapse of time between two formations in
juxtaposition is almost invariably accompanied by a great dissimilarity
in the species of organic remains.
Frequent Unconformability of Strata.—Where the widest gaps appear in
the sequence of the fossil forms, as between the Permian and Triassic
rocks, or between the Cretaceous and Eocene, examples of such
unconformability are very frequent. But they are also met with in some
part or other of the world at the junction of almost all the other
principal formations, and sometimes the subordinate divisions of any
one of the leading groups may be found lying unconformably on another
subordinate member of the same—the Upper, for example, on the Lower
Silurian, or the superior division of the Old Red Sandstone on a lower
member of the same, and so forth. Instances of such irregularities in
the mode of succession of the strata are the more intelligible the more
we extend our survey of the fossiliferous formations, for we are
continually bringing to light deposits of intermediate date, which have
to be intercalated between those previously known, and which reveal to
us a long series of events, of which antecedently to such discoveries
we had no knowledge.
But while unconformability invariably bears testimony to a lapse of
unrepresented time, the conformability of two sets of strata in contact
by no means implies that the newer formation immediately succeeded the
older one. It simply implies that the ancient rocks were subjected to
no movements of such a nature as to tilt, bend, or break them before
the more modern formation was superimposed. It does not show that the
earth’s crust was motionless in the region in question, for there may
have been a gradual sinking or rising, extending uniformly over a large
surface, and yet, during such movement, the stratified rocks may have
retained their original horizontality of position. There may have been
a conversion of a wide area from sea into land and from land into sea,
and during these changes of level some strata may have been slowly
removed by aqueous action, and after this new strata may be
superimposed, differing perhaps in date by thousands of years or
centuries, and yet resting conformably on the older set. There may even
be a blending of the materials constituting the older deposit with
those of the newer, so as to give rise to a passage in the mineral
character of the one rock into the other as if there had been no break
or interruption in the depositing process.
Imperfection of the Record.—Although by the frequent discovery of new
sets of intermediate strata the transition from one type of organic
remains to another is becoming less and less abrupt, yet the entire
series of records appears to the geologists now living far more
fragmentary and defective than it seemed to their predecessors half a
century ago. The earlier inquirers, as often as they encountered a
break in the regular sequence of formations, connected it theoretically
with a sudden and violent catastrophe, which had put an end to the
regular course of events that had been going on uninterruptedly for
ages, annihilating at the same time all or nearly all the organic
beings which had previously flourished, after which, order being
re-established, a new series of events was initiated. In proportion as
our faith in these views grows weaker, and the phenomena of the organic
or inorganic world presented to us by geology seem explicable on the
hypothesis of gradual and insensible changes, varied only by occasional
convulsions, on a scale comparable to that witnessed in historical
times; and in proportion as it is thought possible that former
fluctuations in the organic world may be due to the indefinite
modifiability of species without the necessity of assuming new and
independent acts of creation, the number and magnitude of the gaps
which still remain, or the extreme imperfection of the record, become
more and more striking, and what we possess of the ancient annals of
the earth’s history appears as nothing when contrasted with that which
has been lost.
When we examine a large area such as Europe, the average as well as the
extreme height above the sea attained by the older formations is
usually found to exceed that reached by the more modern ones, the
primary or palaeozoic rising higher than the secondary, and these in
their turn than the tertiary; while in reference to the three divisions
of the tertiary, the lowest or Eocene group attains a higher
summit-level than the Miocene, and these again a greater height than
the Pliocene formations. Lastly, the post-tertiary deposits, such, at
least, as are of marine origin, are most commonly restricted to much
more moderate elevations above the sea-level than the tertiary strata.
It is also observed that strata, in proportion as they are of newer
date, bear the nearest resemblance in mineral character to those which
are now in the progress of formation in seas or lakes, the newest of
all consisting principally of soft mud or loose sand, in some places
full of shells, corals, or other organic bodies, animal or vegetable,
in others wholly devoid of such remains. The farther we recede from the
present time, and the higher the antiquity of the formations which we
examine, the greater are the changes which the sedimentary deposits
have undergone. Time, as I have explained in Chapters V, VI, and VII,
has multiplied the effects of condensation by pressure and cementation,
and the modification produced by heat, fracture, contortion, upheaval,
and denudation. The organic remains also have sometimes been
obliterated entirely, or the mineral matter of which they were composed
has been removed and replaced by other substances.
Why newer Groups should be studied first.—We likewise observe that the
older the rocks the more widely do their organic remains depart from
the types of the living creation. First, we find in the newer tertiary
rocks a few species which no longer exist, mixed with many living ones,
and then, as we go farther back, many genera and families at present
unknown make their appearance, until we come to strata in which the
fossil relics of existing species are nowhere to be detected, except a
few of the lowest forms of invertebrate, while some orders of animals
and plants wholly unrepresented in the living world begin to be
conspicuous.
When we study, therefore, the geological records of the earth and its
inhabitants, we find, as in human history, the defectiveness and
obscurity of the monuments always increasing the remoter the era to
which we refer, and the difficulty of determining the true
chronological relations of rocks is more and more enhanced, especially
when we are comparing those which were formed simultaneously in very
distant regions of the globe. Hence we advance with securer steps when
we begin with the study of the geological records of later times,
proceeding from the newer to the older, or from the more to the less
known.
In thus inverting what might at first seem to be the more natural order
of historical research, we must bear in mind that each of the periods
above enumerated, even the shortest, such as the Post-tertiary, or the
Pliocene, Miocene, or Eocene, embrace a succession of events of vast
extent, so that to give a satisfactory account of what we already know
of any one of them would require many volumes. When, therefore, we
approach one of the newer groups before endeavouring to decipher the
monuments of an older one, it is like endeavouring to master the
history of our own country and that of some contemporary nations,
before we enter upon Roman History, or like investigating the annals of
Ancient Italy and Greece before we approach those of Egypt and Assyria.
Nomenclature.—The origin of the terms Primary and Secondary, and the
synonymous terms Palaeozoic, and Mesozoic, were explained in Chapter
VIII, p. 123.
The Tertiary or Cainozoic strata (see p. 123) were so called because
they were all posterior in date to the Secondary series, of which last
the Chalk of Cretaceous, No. 9, Fig. 86, constitutes the newest group.
The whole of them were at first confounded with the superficial
alluviums of Europe; and it was long before their real extent and
thickness, and the various ages to which they belong, were fully
recognised. They were observed to occur in patches, some of
fresh-water, others of marine origin, their geographical area being
usually small as compared to the secondary formations, and their
position often suggesting the idea of their having been deposited in
different bays, lakes, estuaries, or inland seas, after a large portion
of the space now occupied by Europe had already been converted into dry
land.
The first deposits of this class, of which the characters were
accurately determined, were those occurring in the neighbourhood of
Paris, described in 1810 by MM. Cuvier and Brongniart. They were
ascertained to consist of successive sets of strata, some of marine,
others of fresh-water origin, lying one upon the other. The fossil
shells and corals were perceived to be almost all of unknown species,
and to have in general a near affinity to those now inhabiting warmer
seas. The bones and skeletons of land animals, some of them of large
size, and belonging to more than forty distinct species, were examined
by Cuvier, and declared by him not to agree specifically, nor most of
them even generically, with any hitherto observed in the living
creation.
Strata were soon afterwards brought to light in the vicinity of London,
and in Hampshire, which, although dissimilar in mineral composition,
were justly inferred by Mr. T. Webster to be of the same age as those
of Paris, because the greater number of the fossil shells were
specifically identical. For the same reason, rocks found on the
Gironde, in the South of France, and at certain points in the North of
Italy, were suspected to be of contemporaneous origin.
Another important discovery was soon afterwards made by Brocchi in
Italy, who investigated the argillaceous and sandy deposits, replete
with shells, which form a low range of hills, flanking the Apennines on
both sides, from the plains of the Po to Calabria. These lower hills
were called by him the Subapennines, and were formed of strata chiefly
marine, and newer than those of Paris and London.
Another tertiary group occurring in the neighbourhood of Bordeaux and
Dax, in the South of France, was examined by M. de Basterot in 1825,
who described and figured several hundred species of shells, which
differed for the most part both from the Parisian series and those of
the Subapennine hills. It was soon, therefore, suspected that this
fauna might belong to a period intermediate between that of the
Parisian and Subapennine strata, and it was not long before the
evidence of superposition was brought to bear in support of this
opinion; for other strata, contemporaneous with those of Bordeaux, were
observed in one district (the Valley of the Loire), to overlie the
Parisian formation, and in another (in Piedmont) to underlie the
Subapennine beds. The first example of these was pointed out in 1829 by
M. Desnoyers, who ascertained that the sand and marl of marine origin
called faluns, near Tours, in the basin of the Loire, full of
sea-shells and corals, rested upon a lacustrine formation, which
constitutes the uppermost subdivision of the Parisian group, extending
continuously throughout a great table-land intervening between the
basin of the Seine and that of the Loire. The other example occurs in
Italy, where strata containing many fossils similar to those of
Bordeaux were observed by Bonelli and others in the environs of Turin,
subjacent to strata belonging to the Subapennine group of Brocchi.
Value of Testacean Fossils in Classification.—It will be observed that
in the foregoing allusions to organic remains, the testacea or the
shell-bearing mollusca are selected as the most useful and convenient
class for the purposes of general classification. In the first place,
they are more universally distributed through strata of every age than
any other organic bodies. Those families of fossils which are of rare
and casual occurrence are absolutely of no avail in establishing a
chronological arrangement. If we have plants alone in one group of
strata and the bones of mammalia in another, we can draw no conclusion
respecting the affinity or discordance of the organic beings of the two
epochs compared; and the same may be said if we have plants and
vertebrated animals in one series and only shells in another. Although
corals are more abundant, in a fossil state, than plants, reptiles, or
fish, they are still rare when contrasted with shells, because they are
more dependent for their well-being on the constant clearness of the
water, and are, therefore, less likely to be included in rocks which
endure in consequence of their thickness and the copiousness of
sediment which prevailed when they originated. The utility of the
testacea is, moreover, enhanced by the circumstance that some forms are
proper to the sea, others to the land, and others to fresh water.
Rivers scarcely ever fail to carry down into their deltas some
land-shells, together with species which are at once fluviatile and
lacustrine. By this means we learn what terrestrial, fresh-water, and
marine species coexisted at particular eras of the past: and having
thus identified strata formed in seas with others which originated
contemporaneously in inland lakes, we are then enabled to advance a
step farther, and show that certain quadrupeds or aquatic plants, found
fossil in lacustrine formations, inhabited the globe at the same period
when certain fish, reptiles, and zoophytes lived in the ocean.
Among other characters of the molluscous animals, which render them
extremely valuable in settling chronological questions in geology, may
be mentioned, first, the wide geographical range of many species; and,
secondly, what is probably a consequence of the former, the great
duration of species in this class, for they appear to have surpassed in
longevity the greater number of the mammalia and fish. Had each species
inhabited a very limited space, it could never, when imbedded in
strata, have enabled the geologist to identify deposits at distant
points; or had they each lasted but for a brief period, they could have
thrown no light on the connection of rocks placed far from each other
in the chronological, or, as it is often termed, vertical series.
Classification of Tertiary Strata.—Many authors have divided the
European Tertiary strata into three groups—lower, middle, and upper;
the lower comprising the oldest formations of Paris and London before
mentioned; the middle those of Bordeaux and Touraine; and the upper all
those newer than the middle group.
In the first edition of the Principles of Geology, I divided the whole
of the Tertiary formations into four groups, characterised by the
percentage of recent shells which they contained. The lower tertiary
strata of London and Paris were thought by M. Deshayes to contain only
3½ per cent of recent species, and were termed Eocene. The middle
tertiary of the Loire and Gironde had, according to the specific
determinations of the same conchologist, 17 per cent, and formed the
Miocene division. The Subapennine beds contained 35 to 50 per cent, and
were termed Older Pliocene, while still more recent beds in Sicily,
which had from 90 to 95 per cent of species identical with those now
living, were called Newer Pliocene. The first of the above terms,
Eocene, is derived from eos, _dawn_, and cainos, _recent_, because the
fossil shells of this period contain an extremely small proportion of
living species, which may be looked upon as indicating the dawn of the
existing state of the testaceous fauna, no recent species having been
detected in the older or secondary rocks.
The term Miocene (from meion, _less_, and cainos, _recent_) is intended
to express a minor proportion of recent species (of testacea), the term
Pliocene (from pleion, _ more_, and cainos, _recent_) a comparative
plurality of the same. It may assist the memory of students to remind
them, that the _Mi_ocene contain a _mi_nor proportion, and _ Pl_iocene
a comparative _pl_urality of recent species; and that the greater
number of recent species always implies the more modern origin of the
strata.
It has sometimes been objected to this nomenclature that certain
species of infusoria found in the chalk are still existing, and, on the
other hand, the Miocene and Older Pliocene deposits often contain the
remains of mammalia, reptiles, and fish, exclusively of extinct
species. But the reader must bear in mind that the terms Eocene,
Miocene, and Pliocene were originally invented with reference purely to
conchological data, and in that sense have always been and are still
used by me.
Since the year 1830 the number of known shells, both recent and fossil,
has largely increased, and their identification has been more
accurately determined. Hence some modifications have been required in
the classifications founded on less perfect materials. The Eocene,
Miocene, and Pliocene periods have been made to comprehend certain sets
of strata of which the fossils do not always conform strictly in the
proportion of recent to extinct species with the definitions first
given by me, or which are implied in the etymology of those terms.
CHAPTER X.
RECENT AND POST-PLIOCENE PERIODS.
Recent and Post-pliocene Periods. — Terms defined. — Formations of the
Recent Period. — Modern littoral Deposits containing Works of Art near
Naples. — Danish Peat and Shell-mounds. — Swiss Lake-dwellings. —
Periods of Stone, Bronze, and Iron. — Post-pliocene Formations. —
Coexistence of Man with extinct Mammalia. — Reindeer Period of South of
France. — Alluvial Deposits of Paleolithic Age. — Higher and
Lower-level Valley-gravels. — Loess or Inundation-mud of the Nile,
Rhine, etc. — Origin of Caverns. — Remains of Man and extinct
Quadrupeds in Cavern Deposits. — Cave of Kirkdale. — Australian
Cave-breccias. — Geographical Relationship of the Provinces of living
Vertebrata and those of extinct Post-pliocene Species. — Extinct
struthious Birds of New Zealand. — Climate of the Post-pliocene Period.
— Comparative Longevity of Species in the Mammalia and Testacea. —
Teeth of Recent and Post-pliocene Mammalia.
We have seen in the last chapter that the uppermost or newest strata
are called Post-tertiary, as being more modern than the Tertiary. It
will also be observed that the Post-tertiary formations are divided
into two subordinate groups: the Recent, and Post-pliocene. In the
former, or the Recent, the mammalia as well as the shells are identical
with species now living: whereas in the Post-pliocene, the shells being
all of living forms, a part, and often a considerable part, of the
mammalia belonged to extinct species. To this nomenclature it may be
objected that the term Post-pliocene should in strictness include all
geological monuments posterior in date to the Pliocene; but when I have
occasion to speak of the whole collectively, I shall call them
Post-tertiary, and reserve the term Post-pliocene for the older
Post-tertiary formations, while the Upper or newer ones will be called
“Recent.”
Cases will occur where it may be scarcely possible to draw the boundary
line between the Recent and Post-pliocene deposits; and we must expect
these difficulties to increase rather than diminish with every advance
in our knowledge, and in proportion as gaps are filled up in the series
of records.
RECENT PERIOD
It was stated in the sixth chapter, when I treated of denudation, that
the dry land, or that part of the earth’s surface which is not covered
by the waters of lakes or seas, is generally wasting away by the
incessant action of rain and rivers, and in some cases by the
undermining and removing power of waves and tides on the sea-coast. But
the rate of waste is very unequal, since the level and gently sloping
lands, where they are protected by a continuous covering of vegetation,
escape nearly all wear and tear, so that they may remain for ages in a
stationary condition, while the removal of matter is constantly
widening and deepening the intervening ravines and valleys.
The materials, both fine and coarse, carried down annually by rivers
from the higher regions to the lower, and deposited in successive
strata in the basins of seas and lakes, must be of enormous volume. We
are always liable to underrate their magnitude, because the
accumulation of strata is going on out of sight.
There are, however, causes at work which, in the course of centuries,
tend to render visible these modern formations, whether of marine or
lacustrine origin. For a large portion of the earth’s crust is always
undergoing a change of level, some areas rising and others sinking at
the rate of a few inches, or a few feet, perhaps sometimes yards, in a
century; so that spaces which were once subaqueous are gradually
converted into land, and others which were high and dry become
submerged. In consequence of such movements we find in certain regions,
as in Cashmere, for example, where the mountains are often shaken by
earthquakes, deposits which were formed in lakes in the historical
period, but through which rivers have now cut deep and wide channels.
In lacustrine strata thus intersected, works of art and fresh-water
shells are seen. In other districts on the borders of the sea, usually
at very moderate elevations above its level, raised beaches occur, or
marine littoral deposits, such as those in which, on the borders of the
Bay of Baiæ, near Naples, the well-known temple of Serapis was
imbedded. In that case the date of the monument buried in the marine
strata is ascertainable, but in many other instances the exact age of
the remains of human workmanship is uncertain, as in the estuary of the
Clyde at Glasgow, where many canoes have been exhumed, with other works
of art, all assignable to some part of the Recent Period.
Danish Peat and Shell-mounds or Kitchen-middens.—Sometimes we obtain
evidence, without the aid of a change of level, of events which took
place in pre-historic times. The combined labours, for example, of the
antiquary, zoologist, and botanist have brought to light many monuments
of the early inhabitants buried in peat-mosses in Denmark. Their
geological age is determined by the fact that, not only the
contemporaneous fresh-water and land shells, but all the quadrupeds,
found in the peat, agree specifically with those now inhabiting the
same districts, or which are known to have been indigenous in Denmark
within the memory of man. In the lower beds of peat (a deposit varying
from 20 to 30 feet in thickness), weapons of stone accompany trunks of
the Scotch fir, _Pinus sylvestris._ This peat may be referred to that
part of the stone period for which Sir John Lubbock proposed the name
of “Neolithic”[1] in contradistinction to a still older era, termed by
him “Paleolithic,” and which will be described in the sequel. In the
higher portions of the same Danish bogs, bronze implements are
associated with trunks and acorns of the common oak. It appears that
the pine has never been a native of Denmark in historical times, and it
seems to have given place to the oak about the time when articles and
instruments of bronze superseded those of stone. It also appears that,
at a still later period, the oak itself became scarce, and was nearly
supplanted by the beech, a tree which now flourishes luxuriantly in
Denmark. Again, at the still later epoch when the beech-tree abounded,
tools of iron were introduced, and were gradually substituted for those
of bronze.
On the coasts of the Danish islands in the Baltic, certain mounds,
called in those countries “Kjökken-mödding,” or “kitchen-middens,”
occur, consisting chiefly of the castaway shells of the oyster, cockle,
periwinkle, and other eatable kinds of molluscs. The mounds are from
three to ten feet high, and from 100 to 1000 feet in their longest
diameter. They greatly resemble heaps of shells formed by the Red
Indians of North America along the eastern shores of the United States.
In the old refuse-heaps, recently studied by the Danish antiquaries and
naturalists with great skill and diligence, no implements of metal have
ever been detected. All the knives, hatchets, and other tools, are of
stone, horn, bone, or wood. With them are often intermixed fragments of
rude pottery, charcoal and cinders, and the bones of quadrupeds on
which the rude people fed. These bones belong to wild species still
living in Europe, though some of them, like the beaver, have long been
extirpated in Denmark. The only animal which they seem to have
domesticated was the dog.
As there is an entire absence of metallic tools, these refuse-heaps are
referred to the Neolithic division of the age of stone, which
immediately preceded in Denmark the age of bronze. It appears that a
race more advanced in civilisation, armed with weapons of that mixed
metal, invaded Scandinavia, and ousted the aborigines.
Lacustrine Habitations of Switzerland.—In Switzerland a different class
of monuments, illustrating the successive ages of stone, bronze, and
iron, has been of late years investigated with great success, and
especially since 1854, in which year Dr. F. Keller explored near the
shore at Meilen, in the bottom of the lake of Zurich, the ruins of an
old village, originally built on numerous wooden piles, driven, at some
unknown period, into the muddy bed of the lake. Since then a great many
other localities, more than a hundred and fifty in all, have been
detected of similar pile-dwellings, situated near the borders of the
Swiss lakes, at points where the depth of water does not exceed 15
feet.[2] The superficial mud in such cases is filled with various
articles, many hundreds of them being often dredged up from a very
limited area. Thousands of piles, decayed at their upper extremities,
are often met with still firmly fixed in the mud.
As the ages of stone, bronze, and iron merely indicate successive
stages of civilisation, they may all have coexisted at once in
different parts of the globe, and even in contiguous regions, among
nations having little intercourse with each other. To make out,
therefore, a distinct chronological series of monuments is only
possible when our observations are confined to a limited district, such
as Switzerland.
The relative antiquity of the pile-dwellings, which belong respectively
to the ages of stone and bronze, is clearly illustrated by the
associations of the tools with certain groups of animal remains. Where
the tools are of stone, the castaway bones which served for the food of
the ancient people are those of deer, the wild boar, and wild ox, which
abounded when society was in the hunter state. But the bones of the
later or bronze epoch were chiefly those of the domestic ox, goat, and
pig, indicating progress in civilisation. Some villages of the stone
age are of later date than others, and exhibit signs of an improved
state of the arts. Among their relics are discovered carbonised grains
of wheat and barley, and pieces of bread, proving that the cultivation
of cereals had begun. In the same settlements, also, cloth, made of
woven flax and straw, has been detected.
The pottery of the bronze age in Switzerland is of a finer texture, and
more elegant in form, than that of the age of stone. At Nidau, on the
lake of Bienne, articles of iron have also been discovered, so that
this settlement was evidently not abandoned till that metal had come
into use.
At La Thène, in the northern angle of the lake of Neufchâtel, a great
many articles of iron have been obtained, which in form and
ornamentation are entirely different both from those of the bronze
period and from those used by the Romans. Gaulish and Celtic coins have
also been found there by MM. Schwab and Desor. They agree in character
with remains, including many iron swords, which have been found at
Tiefenau, near Berne, in ground supposed to have been a battle-field;
and their date appears to have been anterior to the great Roman
invasion of Northern Europe, though perhaps not long before that
event.[3] Coins, which sometimes occur in deposits of the age of iron,
have never yet been found in formations of the ages of bronze or stone.
The period of bronze must have been one of foreign commerce, as tin,
which enters into this metallic mixture in the proportion of about ten
per cent to the copper, was obtained by the ancients chiefly from
Cornwall.[4] Very few human bones of the bronze period have been met
with in the Danish peat, or in the Swiss lake-dwellings, and this
scarcity is generally attributed by archæologists to the custom of
burning the dead, which prevailed in the age of bronze.
POST-PLIOCENE PERIOD
From the foregoing observations we may infer that the ages of iron and
bronze in Northern and Central Europe were preceded by a stone age, the
Neolithic, referable to that division of the post-tertiary epoch which
I have called Recent, when the mammalia as well as the other organic
remains accompanying the stone implements were of living species. But
memorials have of late been brought to light of a still older age of
stone, for which, as above stated, the name Paleolithic has been
proposed, when man was contemporary in Europe with the elephant and
rhinoceros, and various other animals, of which many of the most
conspicuous have long since died out.
Reindeer Period in South of France.—In the larger number of the caves
of Europe, as for example in those of England, Belgium, Germany, and
many parts of France, the animal remains agree specifically with the
fauna of this oldest division of the age of stone, or that to which
belongs the drift of Amiens and Abbeville presently to be mentioned,
containing flint implements of a very antique type. But there are some
caves in the departments of Dordogne, Aude, and other parts of the
south of France, which are believed by M. Lartet to be of intermediate
date between the Paleolithic and Neolithic periods. To this
intermediate era M. Lartet gave, in 1863, the name of the “reindeer
period,” because vast quantities of the bones and horns of that deer
have been met with in the French caverns. In some cases separate plates
of molars of the mammoth, and several teeth of the great Irish deer,
_Cervus megaceros,_ and of the cave-lion, _Felis spelæa,_ have been
found mixed up with cut and carved bones of reindeer. On one of these
sculptured bones in the cave of Perigord, a rude representation of the
mammoth, with its long curved tusks and covering of wool, occurs, which
is regarded by M. Lartet as placing beyond all doubt the fact that the
early inhabitants of these caves must have seen this species of
elephant still living in France. The presence of the marmot, as well as
the reindeer and some other northern animals, in these caverns seems to
imply a colder climate than that of the Swiss lake-dwellings, in which
no remains of reindeer have as yet been discovered. The absence of this
last in the old lacustrine habitations of Switzerland is the more
significant, because in a cave in the neighbourhood of the lake of
Geneva, namely, that of Mont Saleve, bones of the reindeer occur with
flint implements similar to those of the caverns of Dordogne and
Perigord.
The state of the arts, as exemplified by the instruments found in these
caverns of the reindeer period, is somewhat more advanced than that
which characterises the tools of the Amiens drift, but is nevertheless
more rude than that of the Swiss lake-dwellings. No metallic articles
occur, and the stone hatchets are not ground after the fashion of
celts; the needles of bone are shaped in a workmanlike style, having
their eyes drilled with consummate skill.
The formations above alluded to, which are as yet but imperfectly
known, may be classed as belonging to the close of the Paleolithic era,
of the monuments of which I am now about to treat.
Alluvial Deposits of the Paleolithic Age.—The alluvial and marine
deposits of the Paleolithic age, the earliest to which any vestiges of
man have yet been traced back, belong to a time when the physical
geography of Europe differed in a marked degree from that now
prevailing. In the Neolithic period, the valleys and rivers coincided
almost entirely with those by which the present drainage of the land is
effected, and the peat-mosses were the same as those now growing. The
situation of the shell-mounds and lake-dwellings above alluded to is
such as to imply that the topography of the districts where they are
observed has not subsequently undergone any material alteration.
Whereas we no sooner examine the Post-pliocene formations, in which the
remains of so many extinct mammalia are found, than we at once perceive
a more decided discrepancy between the former and present outline of
the surface. Since those deposits originated, changes of considerable
magnitude have been effected in the depth and width of many valleys, as
also in the direction of the superficial and subterranean drainage,
and, as is manifest near the sea-coast, in the relative position of
land and water. In Fig. 87 an ideal section is given, illustrating the
different position which the Recent and Post-pliocene alluvial deposits
occupy in many European valleys.
Fig. 87: Recent and Post-pliocene alluvial deposits.
The peat, No. 1, has been formed in a low part of the modern alluvial
plain, in parts of which gravel No. 2 of the recent period is seen.
Over this gravel the loam or fine sediment 2′ has in many places been
deposited by the river during floods which covered nearly the whole
alluvial plain.
No. 3 represents an older alluvium, composed of sand and gravel, formed
before the valley had been excavated to its present depth. It contains
the remains of fluviatile shells of living species associated with the
bones of mammalia, in part of recent, and in part of extinct species.
Among the latter, the mammoth (_E. primigenius_) and the Siberian
rhinoceros (_R. tichorhinus_) are the most common in Europe. No. 3′ is
a remnant of the loam or brick-earth by which No. 3 was overspread. No.
4 is a still older and more elevated terrace, similar in its
composition and organic remains to No. 3, and covered in like manner
with its inundation-mud, 4′. Sometimes the valley-gravels of older date
are entirely missing, or there is only one, and occasionally there are
more than two, marking as many successive stages in the excavation of
the valley. They usually occur at heights varying from 10 to 100 feet,
sometimes on the right and sometimes on the left side of the existing
river-plain, but rarely in great strength on exactly opposite sides of
the valley.
Among the genera of extinct quadrupeds most frequently met with in
England, France, Germany, and other parts of Europe, are the elephant,
rhinoceros, hippopotamus, horse, great Irish deer, bear, tiger, and
hyæna. In the peat, No. 1 (Fig. 87), and in the more modern gravel and
silt (No. 2), works of art of the ages of iron and bronze, and of the
later or Neolithic stone period, already described, are met with. In
the more ancient or Paleolithic gravels, 3 and 4, there have been found
of late years in several valleys in France and England—as, for example,
in those of the Seine and Somme, and of the Thames and Ouse, near
Bedford—stone implements of a rude type, showing that man coexisted in
those districts with the mammoth and other extinct quadrupeds of the
genera above enumerated. In 1847, M. Boucher de Perthes observed in an
ancient alluvium at Abbeville, in Picardy, the bones of extinct
mammalia associated in such a manner with flint implements of a rude
type as to lead him to infer that both the organic remains and the
works of art were referable to one and the same period. This inference
was soon after confirmed by Mr. Prestwich, who found in 1859 a flint
tool in situ in the same stratum at Amiens that contained the remains
of extinct mammalia.
The flint implements found at Abbeville and Amiens are most of them
considered to be hatchets and spear-heads, and are different from those
commonly called “celts.” These celts, so often found in the recent
formations, have a more regular oblong shape, the result of grinding,
by which also a sharp edge has been given to them. The Abbeville tools
found in gravel at different levels, as in Nos. 3 and 4, Fig. 87, in
which bones of the elephant, rhinoceros, and other extinct mammalia
occur, are always unground, having evidently been brought into their
present form simply by the chipping off of fragments of flint by
repeated blows, such as could be given by a stone hammer.
Some of them are oval, others of a spear-headed form, no two exactly
alike, and yet the greater number of each kind are obviously fashioned
after the same general pattern. Their outer surface is often white, the
original black flint having been discoloured and bleached by exposure
to the air, or by the action of acids, as they lay in the gravel. They
are most commonly stained of the same ochreous colour as the flints of
the gravel in which they are imbedded. Occasionally their antiquity is
indicated not only by their colour but by superficial incrustations of
carbonate of lime, or by dendrites formed of oxide of iron and
manganese. The edges also of most of them are worn, sometimes by having
been used as tools, or sometimes by having been rolled in the old
river’s bed. They are met with not only in the lower-level gravels, as
in No. 3, Fig. 87, but also in No. 4, or the higher gravels, as at St.
Acheul, in the suburbs of Amiens, where the old alluvium lies at an
elevation of about 100 feet above the level of the river Somme. At both
levels fluviatile and land-shells are met with in the loam as well as
in the gravel, but there are no marine shells associated, except at
Abbeville, in the lowest part of the gravel, near the sea, and a few
feet only above the present high-water mark. Here with fossil shells of
living species are mingled the bones of _Elephas primigenius_ and _E.
antiquus, Rhinoceros tichorhinus, Hippopotamus, Felis spelæa, Hyæna
spelæa,_ reindeer, and many others, the bones accompanying the flint
implements in such a manner as to show that both were buried in the old
alluvium at the same period.
Nearly the entire skeleton of a rhinoceros was found at one point,
namely, in the Menchecourt drift at Abbeville, the bones being in such
juxtaposition as to show that the cartilage must have held them
together at the time of their inhumation.
The general absence here and elsewhere of human bones from gravel and
sand in which flint tools are discovered, may in some degree be due to
the present limited extent of our researches. But it may also be
presumed that when a hunter population, always scanty in numbers,
ranged over this region, they were too wary to allow themselves to be
overtaken by the floods which swept away many herbivorous animals from
the low river-plains where they may have been pasturing or sleeping.
Beasts of prey prowling about the same alluvial flats in search of food
may also have been surprised more readily than the human tenant of the
same region, to whom the signs of a coming tempest were better known.
Inundation-mud of Rivers.—Brick-earth.—Fluviatile Loam, or Loess.—As a
general rule, the fluviatile alluvia of different ages (Nos. 2, 3, 4,
Fig. 87) are severally made up of coarse materials in their lower
portions, and of fine silt or loam in their upper parts. For rivers are
constantly shifting their position in the valley-plain, encroaching
gradually on one bank, near which there is deep water, and deserting
the other or opposite side, where the channel is growing shallower,
being destined eventually to be converted into land. Where the current
runs strongest, coarse gravel is swept along, and where its velocity is
slackened, first sand, and then only the finest mud, is thrown down. A
thin film of this fine sediment is spread, during floods, over a wide
area, on one, or sometimes on both sides, of the main stream, often
reaching as far as the base of the bluffs or higher grounds which bound
the valley. Of such a description are the well-known annual deposits of
the Nile, to which Egypt owes its fertility. So thin are they, that the
aggregate amount accumulated in a century is said rarely to exceed five
inches, although in the course of thousands of years it has attained a
vast thickness, the bottom not having been reached by borings extending
to a depth of 60 feet towards the central parts of the valley.
Everywhere it consists of the same homogeneous mud, destitute of
stratification—the only signs of successive accumulation being where
the Nile has silted up its channel, or where the blown sands of the
Libyan desert have invaded the plain, and give rise to alternate layers
of sand and mud.
In European river-loams we occasionally observe isolated pebbles and
angular pieces of stone which have been floated by ice to the places
where they now occur; but no such coarse materials are met with in the
plains of Egypt.
In some parts of the valley of the Rhine the accumulation of similar
loam, called in Germany “loess,” has taken place on an enormous scale.
Its colour is yellowish-grey, and very homogeneous; and Professor
Bischoff has ascertained, by analysis, that it agrees in composition
with the mud of the Nile. Although for the most part unstratified, it
betrays in some places marks of stratification, especially where it
contains calcareous concretions, or in its lower part where it rests on
subjacent gravel and sand which alternate with each other near the
junction. About a sixth part of the whole mass is composed of carbonate
of lime, and there is usually an intermixture of fine quartzose and
micaceous sand.
Although this loam of the Rhine is unsolidified, it usually terminates
where it has been undermined by running water in a vertical cliff, from
the face of which shells of terrestrial, fresh-water and amphibious
mollusks project in relief. These shells do not imply the permanent
sojourn of a body of fresh water on the spot, for the most aquatic of
them, the _Succinea_, inhabits marshes and wet grassy meadows. The
_Succinea elongata_ (or _S. oblongata_), Fig. 88, is very
characteristic both of the loess of the Rhine and of some other
European river-loams.
Fig. 88: Succinea elongata; Fig. 89: Pupa muscorum (Linn.); Fig. 90:
Helix hispida (Linn.) (plebia).
Among the land-shells of the Rhenish loess, _Helix hispida_, Fig. 90,
and _Pupa muscorum_, Fig. 89, are very common. Both the terrestrial and
aquatic shells are of most fragile and delicate structure, and yet they
are almost invariably perfect and uninjured. They must have been broken
to pieces had they been swept along by a violent inundation. Even the
colour of some of the land-shells, as that of _Helix nemoralis_, is
occasionally preserved.
In parts of the valley of the Rhine, between Bingen and Basle, the
fluviatile loam or loess now under consideration is several hundred
feet thick, and contains here and there throughout that thickness land
and amphibious shells. As it is seen in masses fringing both sides of
the great plain, and as occasionally remnants of it occur in the centre
of the valley, forming hills several hundred feet in height, it seems
necessary to suppose, first, a time when it slowly accumulated; and
secondly, a later period, when large portions of it were removed, or
when the original valley, which had been partially filled up with it,
was re-excavated.
Such changes may have been brought about by a great movement of
oscillation, consisting first of a general depression of the land, and
then of a gradual re-elevation of the same. The amount of continental
depression which first took place in the interior, must be imagined to
have exceeded that of the region near the sea, in which case the higher
part of the great valley would have its alluvial plain gradually raised
by an accumulation of sediment, which would only cease when the
subsidence of the land was at an end. If the direction of the movement
was then reversed, and, during the re-elevation of the continent, the
inland region nearest the mountains should rise more rapidly than that
near the coast, the river would acquire a denuding power sufficient to
enable it to sweep away gradually nearly all the loam and gravel with
which parts of its basin had been filled up. Terraces and hillocks of
mud and sand would then alone remain to attest the various levels at
which the river had thrown down and afterwards removed alluvial matter.
Cavern Deposits containing Human Remains and Bones of Extinct
Animals.—In England, and in almost all countries where limestone rocks
abound, caverns are found, usually consisting of cavities of large
dimensions, connected together by low, narrow, and sometimes torturous
galleries or tunnels. These subterranean vaults are usually filled in
part with mud, pebbles, and breccia, in which bones occur belonging to
the same assemblage of animals as those characterising the
Post-pliocene alluvia above described. Some of these bones are
referable to extinct and others to living species, and they are
occasionally intermingled, as in the valley-gravels, with implements of
one or other of the great divisions of the stone age, and these are not
unfrequently accompanied by human bones, which are much more common in
cavern deposits than in valley-alluvium.
Each suite of caverns, and the passages by which they communicate the
one with the other, afford memorials to the geologist of successive
phases through which they must have passed. First, there was a period
when the carbonate of lime was carried out gradually by springs;
secondly, an era when engulfed rivers or occasional floods swept
organic and inorganic debris into the subterranean hollows previously
formed; and thirdly, there were such changes in the configuration of
the region as caused the engulfed rivers to be turned into new
channels, and springs to be dried up, after which the cave-mud,
breccia, gravel, and fossil bones would bear the same kind of relation
to the existing drainage of the country as the older valley-drifts with
their extinct mammalian remains and works of art bear to the present
rivers and alluvial plains.
The quarrying away of large masses of Carboniferous and Devonian
limestone, near Liege, in Belgium, has afforded the geologist
magnificent sections of some of these caverns, and the former
communication of cavities in the interior of the rocks with the old
surface of the country by means of vertical or oblique fissures, has
been demonstrated in places where it would not otherwise have been
suspected, so completely have the upper extremities of these fissures
been concealed by superficial drift, while their lower ends, which
extended into the roofs of the caves, are masked by stalactitic
incrustations.
The origin of the stalactite is thus explained by the eminent chemist
Liebig. Mould or humus, being acted on by moisture and air, evolves
carbonic acid, which is dissolved by rain. The rain-water, thus
impregnated, permeates the porous limestone, dissolves a portion of it,
and afterwards, when the excess of carbonic acid evaporates in the
caverns, parts with the calcareous matter, and forms stalactite. Even
while caverns are still liable to be occasionally flooded such
calcareous incrustations accumulate, but it is generally when they are
no longer in the line of drainage that a solid floor of hard stalagmite
is formed on the bottom.
The late Dr. Schmerling examined forty caves near Liege, and found in
all of them the remains of the same fauna, comprising the mammoth,
tichorhine rhinoceros, cave-bear, cave-hyæna, cave-lion, and many
others, some of extinct and some of living species, and in all of them
flint implements. In four or five caves only parts of human skeletons
were met with, comprising sometimes skulls with a few other bones,
sometimes nearly every part of the skeleton except the skull. In one of
the caves, that of Engihoul, where Schmerling had found the remains of
at least three human individuals, they were mingled in such a manner
with bones of extinct mammalia, as to leave no doubt on his mind (in
1833) of man having co-existed with them.
In 1860, Professor Malaise, of Liege, explored with me this same cave
of Engihoul, and beneath a hard floor of stalagmite we found mud full
of bones of extinct and recent animals, such as Schmerling had
described, and my companion, persevering in his researches after I had
returned to England, extracted from the same deposit two human lower
jaw-bones retaining their teeth. The skulls from these Belgian caverns
display no marked deviation from the normal European type of the
present day.
The careful investigations carried on by Dr. Falconer, Mr. Pengelly,
and others, in the Brixham cave near Torquay, in 1858, demonstrated
that flint knives were there imbedded in such a manner in loam
underlying a floor of stalagmite as to prove that man had been an
inhabitant of that region when the cave-bear and other members of the
ancient post-pliocene fauna were also in existence.
The absence of gnawed bones had led Dr. Schmerling to infer that none
of the Belgian caves which he explored had served as the dens of wild
beasts; but there are many caves in Germany and England which have
certainly been so inhabited, especially by the extinct hyæna and bear.
A fine example of a hyæna’s den was afforded by the cave of Kirkdale,
so well described by the late Dr. Buckland in his _Reliquiæ Diluvianæ._
In that cave, above twenty-five miles north-north-east of York, the
remains of about 300 hyænas, belonging to individuals of every age,
were detected. The species (_Hyæna spelæa_) has been considered by
palæontologists as extinct; it was larger than the fierce _Hyæna
crocuta_ of South Africa, which it closely resembled, and of which it
is regarded by Mr. Boyd Dawkins as a variety. Dr. Buckland, after
carefully examining the spot, proved that the hyænas must have lived
there; a fact attested by the quantity of their dung, which, as in the
case of the living hyæna, is of nearly the same composition as bone,
and almost as durable. In the cave were found the remains of the ox,
young elephant, hippopotamus, rhinoceros, horse, bear, wolf, hare,
water-rat, and several birds. All the bones have the appearance of
having been broken and gnawed by the teeth of the hyænas; and they
occur confusedly mixed in loam or mud, or dispersed through a crust of
stalagmite which covers it. In these and many other cases it is
supposed that portions of herbivorous quadrupeds have been dragged into
caverns by beasts of prey, and have served as their food—an opinion
quite consistent with the known habits of the living hyæna.
_Australian Cave-breccias._—Ossiferous breccias are not confined to
Europe, but occur in all parts of the globe; and those discovered in
fissures and caverns in Australia correspond closely in character with
what has been called the bony breccia of the Mediterranean, in which
the fragments of bone and rock are firmly bound together by a red
ochreous cement.
Fig. 91: Part of a lower jaw of Macropus atlas.
Some of these caves were examined by the late Sir T. Mitchell in the
Wellington Valley, about 210 miles west of Sidney, on the river Bell,
one of the principal sources of the Macquarie, and on the Macquarie
itself. The caverns often branch off in different directions through
the rock, widening and contracting their dimensions, and the roofs and
floors are covered with stalactite. The bones are often broken, but do
not seem to be water-worn. In some places they lie imbedded in loose
earth, but they are usually included in a breccia.
The remains belong to marsupial animals. Among the most abundant are
those of the kangaroo, of which there are four species, while others
belong to the genera _Phascolomys_, the wombat; _Dasyurus_), the ursine
opossum; _Phalangista_, the vulpine opossum; and _Hypsiprymnus_, the
kangaroo-rat.
Fig. 92: Lower jaw of largest living species of kangaroo.
In the fossils above enumerated, several species are larger than the
largest living ones of the same genera now known in Australia. Fig. 91
of the right side of a lower jaw of a kangaroo (_Macropus atlas_, Owen)
will at once be seen to exceed in magnitude the corresponding part of
the largest living kangaroo, which is represented in Fig. 92. In both
these specimens part of the substance of the jaw has been broken open,
so as to show the permanent false molar (_a_, Fig. 91), concealed in
the socket. From the fact of this molar not having been cut, we learn
that the individual was young, and had not shed its first teeth.
The reader will observe that all these extinct quadrupeds of Australia
belong to the marsupial family, or, in other words, that they are
referable to the same peculiar type of organisation which now
distinguishes the Australian mammalia from those of other parts of the
globe. This fact is one of many pointing to a general law deducible
from the fossil vertebrate and invertebrate animals of times
immediately antecedent to our own, namely, that the present
geographical distribution of organic _forms_ dates back to a period
anterior to the origin of existing _species_; in other words, the
limitation of particular genera or families of quadrupeds, mollusca,
etc., to certain existing provinces of land and sea, began before the
larger part of the species now contemporary with man had been
introduced into the earth.
Professor Owen, in his excellent “History of British Fossil Mammals,”
has called attention to this law, remarking that the fossil quadrupeds
of Europe and Asia differ from those of Australia or South America. We
do not find, for example, in the Europæo-Asiatic province fossil
kangaroos, or armadillos, but the elephant, rhinoceros, horse, bear,
hyæna, beaver, hare, mole, and others, which still characterise the
same continent.
In like manner, in the Pampas of South America the skeletons of
Megatherium, Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and
other extinct forms, are analogous to the living sloth, armadillo,
cavy, capybara, and llama. The fossil quadrumana, also associated with
some of these forms in the Brazilian caves, belong to the Platyrrhine
family of monkeys, now peculiar to South America. That the extinct
fauna of Buenos Ayres and Brazil was very modern has been shown by its
relation to deposits of marine shells, agreeing with those now
inhabiting the Atlantic.
The law of geographical relationship above alluded to, between the
living vertebrata of every great zoological province and the fossils of
the period immediately antecedent, even where the fossil species are
extinct, is by no means confined to the mammalia. New Zealand, when
first examined by Europeans, was found to contain no indigenous land
quadrupeds, no kangaroos, or opossums, like Australia; but a wingless
bird abounded there, the smallest living representative of the ostrich
family, called the Kiwi by the natives (_Apteryx_). In the fossils of
the Post-pliocene period in this same island, there is the like absence
of kangaroos, opossums, wombats, and the rest; but in their place a
prodigious number of well-preserved specimens of gigantic birds of the
struthious order, called by Owen _Dinornis_ and _Palapteryx_, which are
entombed in superficial deposits. These genera comprehended many
species, some of which were four, some seven, others nine, and others
eleven feet in height! It seems doubtful whether any contemporary
mammalia shared the land with this population of gigantic feathered
bipeds.
Mr. Darwin, when describing the recent and fossil mammalia of South
America, has dwelt much on the wonderful relationship of the extinct to
the living types in that part of the world, inferring from such
geographical phenomena that the existing species are all related to the
extinct ones which preceded them by a bond of common descent.
Climate of the Post-pliocene Period.—The evidence as to the climate of
Europe during this epoch is somewhat conflicting. The fluviatile and
land-shells are all of existing species, but their geographical range
has not always been the same as at present. Some, for example, which
then lived in Britain are now only found in Norway and Finland,
probably implying that the Post-pliocene climate of Britain was colder,
especially in the winter. So also the reindeer and the musk-ox (_Ovibos
moschatus_), now inhabitants of the Arctic regions, occur fossil in the
valleys of the Thames and Avon, and also in France and Germany,
accompanied in most places by the mammoth and the woolly rhinoceros. At
Grays in Essex, on the other hand, another species both of elephant and
rhinoceros occurs, together with a hippopotamus and the _Cyrena
fluminalis_, a shell now extinct in Europe but still an inhabitant of
the Nile and some Asiatic rivers. With it occurs the _Unio littoralis_,
now living in the Seine and Loire. In the valley of the Somme flint
tools have been found associated with _Hippopotamus major_ and _Cyrena
fluminalis_ in the lower-level Post-pliocene gravels; while in the
higher-level (and more ancient) gravels similar tools are more
abundant, and are associated with the bones of the mammoth and other
Post-pliocene quadrupeds indicative of a colder climate.
It is possible that we may here have evidence of summer and winter
migrations rather than of a general change of temperature. Instead of
imagining that the hippopotamus lived all the year round with the
musk-ox and lemming, we may rather suppose that the apparently
conflicting evidence may be due to the place of our observations being
near the boundary line of a northern and southern fauna, either of
which may have advanced or receded during comparatively slight and
temporary fluctuations of climate. There may then have been a
continuous land communication between England and the North of Siberia,
as well as in an opposite direction with Africa, then united to
Southern Europe.
In drift at Fisherton, near Salisbury, thirty feet above the river
Wiley, the Greenland lemming and a new species of the Arctic genus
_Spermophilus_ have been found, along with the mammoth, reindeer,
cave-hyæna, and other mammalia suited to a cold climate. A flint
implement was taken out from beneath the bones of the mammoth. In a
higher and older deposit in the vicinity, flint tools like those of
Amiens have been discovered. Nearly all the known Post-pliocene
quadrupeds have now been found accompanying flint knives or hatchets in
such a way as to imply their coexistence with man; and we have thus the
concurrent testimony of several classes of geological facts to the vast
antiquity of the human race. In the first place, the disappearance of a
great variety of species of wild animals from every part of a wide
continent must have required a vast period for its accomplishment; yet
this took place while man existed upon the earth, and was completed
before that early period when the Danish shell-mounds were formed or
the oldest of the Swiss lake-dwellings constructed. Secondly, the
deepening and widening of valleys, indicated by the position of the
river gravels at various heights, implies an amount of change of which
that which has occurred during the historical period forms a scarcely
perceptible part. Thirdly, the change in the course of rivers which
once flowed through caves now removed from any line of drainage, and
the formation of solid floors of stalagmite, must have required a great
lapse of time. Lastly, ages must have been required to change the
climate of wide regions to such an extent as completely to alter the
geographical distribution of many mammalia as well as land and
fresh-water shells. The 3000 or 4000 years of the historical period
does not furnish us with any appreciable measure for calculating the
number of centuries which would suffice for such a series of changes,
which are by no means of a local character, but have operated over a
considerable part of Europe.
Relative Longevity of Species in the Mammalia and Testacea.—I called
attention in 1830[5] to the fact, which had not at that time attracted
notice, that the association in the Post-pliocene deposits of shells,
exclusively of living species, with many extinct quadrupeds betokened a
longevity of species in the testacea far exceeding that in the
mammalia. Subsequent researches seem to show that this greater duration
of the same specific forms in the class mollusca is dependent on a
still more general law, namely, that the lower the grade of animals, or
the greater the simplicity of their structure, the more persistent are
they in general in their specific characters throughout vast periods of
time. Not only have the invertebrata, as shown by geological data,
altered at a less rapid rate than the vertebrata, but if we take one of
the classes of the former, as for example the mollusca, we find those
of more simple structure to have varied at a slower rate than those of
a higher and more complex organisation; the Brachiopoda, for example,
more slowly than the lamellibranchiate bivalves, while the latter have
been more persistent than the univalves, whether gasteropoda or
cephalopoda. In like manner the specific identity of the characters of
the foraminifera, which are among the lowest types of the invertebrata,
has outlasted that of the mollusca in an equally decided manner.
Teeth of Post-pliocene Mammalia.—To those who have never studied
comparative anatomy, it may seem scarcely credible that a single bone
taken from any part of the skeleton may enable a skilful osteologist to
distinguish, in many cases, the genus, and sometimes the species, of
quadrupeds to which it belonged. Although few geologists can aspire to
such knowledge, which must be the result of long practice and study,
they will nevertheless derive great advantage from learning, what is
comparatively an easy task, to distinguish the principal divisions of
the mammalia by the forms and characters of their teeth.
Fig. 93: Elephas primigenius (or Mammoth) molar of upper jaw, right
side. Post-pliocene; Fig. 94: Elephas antiquus, Falconer. Penultimate
molar. Post-pliocene and Pliocene.
Figures 93 through 105 represent the teeth of some of the more common
species and genera found in alluvial and cavern deposits.
Figs. 95 to 100: Teeth of extinct mammalia.
Figs. 101 to 105: Teeth of extinct mammalia.
On comparing the grinding surfaces of the corresponding molars of the
three species of elephants, Figs. 93, 94, 95 it will be seen that the
folds of enamel are most numerous in the mammoth, fewer and wider, or
more open, in _E. antiquus_; and most open and fewest in _E.
meridionalis._ It will be also seen that the enamel in the molar of the
_Rhinoceros tichorhinus_ (Fig. 97), is much thicker than in that of the
_Rhinoceros leptorhinus_ (Fig. 96).
[1] Sir John Lubbock, Pre-historic Times, p. 3, 1865.
[2] Bulletin de la Sociétié Vaudoise des Sci. Nat., tome vi, Lausanne
1860; and Antiquity of Man, by the author, chap. ii.
[3] Sir J. Lubbock’s Lecture, Royal Institution, Feb. 27th, 1863.
[4] Diodorus, v, 21, 22 and Sir H. James Note on Block of Tin dredged
up in Falmouth Harbour. Royal Institution of Cornwall, 1863.
[5] Principles of Geology, 1st ed., vol. iii, p. 140.
CHAPTER XI.
POST-PLIOCENE PERIOD, continued—GLACIAL CONDITIONS.[1]
Geographical Distribution, Form, and Characters of Glacial Drift. —
Fundamental Rocks, polished, grooved, and scratched. — Abrading and
striating Action of Glaciers. — Moraines, Erratic Blocks, and “Roches
Moutonnees.” Alpine Blocks on the Jura. — Continental Ice of Greenland.
— Ancient Centres of the Dispersion of Erratics. — Transportation of
Drift by floating Icebergs. — Bed of the Sea furrowed and polished by
the running aground of floating Ice-islands.
Character and Distribution of Glacial Drift.—In speaking of the loose
transported matter commonly found on the surface of the land in all
parts of the globe, I alluded to the exceptional character of what has
been called the boulder formation in the temperate and Arctic latitudes
of the northern hemisphere. The peculiarity of its form in Europe north
of the 50th, and in North America north of the 40th parallel of
latitude, is now universally attributed to the action of ice, and the
difference of opinion respecting it is now chiefly restricted to the
question whether land-ice or floating icebergs have played the chief
part in its distribution. It is wanting in the warmer and equatorial
regions, and reappears when we examine the lands which lie south of the
40th and 50th parallels in the southern hemisphere, as, for example, in
Patagonia, Tierra del Fuego, and New Zealand. It consists of sand and
clay, sometimes stratified, but often wholly devoid of stratification
for a depth of 50, 100, or even a greater number of feet. To this
unstratified form of the deposit the name of _till_ has long been
applied in Scotland. It generally contains a mixture of angular and
rounded fragments of rock, some of large size, having occasionally one
or more of their sides flattened and smoothed, or even highly polished.
The smoothed surfaces usually exhibit many scratches parallel to each
other, one set of which often crosses an older set. The till is almost
everywhere wholly devoid of organic remains, except those washed into
it from older formations, though in some places it contains marine
shells, usually of northern or Arctic species, and frequently in a
fragmentary state. The bulk of the till has usually been derived from
the grinding down into mud of rocks in the immediate neighbourhood, so
that it is red in a region of Red Sandstone, as in Strathmore in
Forfarshire; grey or black in a district of coal and bituminous shale,
as around Edinburgh; and white in a chalk country, as in parts of
Norfolk and Denmark. The stony fragments dispersed irregularly through
the till usually belong, especially in mountainous countries, to rocks
found in some part of the same hydrographical basin; but there are
regions where the whole of the boulder clay has come from a distance,
and huge blocks, or “erratics,” as they have been called, many feet in
diameter, have not unfrequently travelled hundreds of miles from their
point of departure, or from the parent rocks from which they have
evidently been detached. These are commonly angular, and have often one
or more of their sides polished and furrowed.
The rock on which the boulder formation reposes, if it consists of
granite, gneiss, marble, or other hard stone, capable of permanently
retaining any superficial markings which may have been imprinted upon
it, is usually smoothed or polished, like the erratics above described,
and exhibits parallel striæ and furrows having a determinate direction.
This direction, both in Europe and North America, agrees generally in a
marked manner with the course taken by the erratic blocks in the same
district. The boulder clay, when it was first studied, seemed in many
of its characters so singular and anomalous, that geologists despaired
of ever being able to interpret the phenomena by reference to causes
now in action. In those exceptional cases where marine shells of the
same date as the boulder clay were found, nearly all of them were
recognised as living species—a fact conspiring with the superficial
position of the drift to indicate a comparatively modern origin.
The term “diluvium” was for a time the most popular name of the boulder
formation, because it was referred by many to the deluge of Noah, while
others retained the name as expressive of their opinion that a series
of diluvial waves raised by hurricanes and storms, or by earthquakes,
or by the sudden upheaval of land from the bed of the sea, had swept
over the continents, carrying with them vast masses of mud and heavy
stones, and forcing these stones over rocky surfaces so as to polish
and imprint upon them long furrows and striæ. But geologists were not
long in seeing that the boulder formation was characteristic of high
latitudes, and that on the whole the size and number of erratic blocks
increases as we travel towards the Arctic regions. They could not fail
to be struck with the contrast which the countries bordering the Baltic
presented when compared with those surrounding the Mediterranean. The
multitude of travelled blocks and striated rocks in the one region, and
the absence of such appearances in the other, were too obvious to be
overlooked. Even the great development of the boulder formation, with
large erratics so far south as the Alps, offered an exception to the
general rule favourable to the hypothesis that there was some intimate
connection between it and accumulations of snow and ice.
Fig. 106: Limestone, polished, furrowed, and scratched by the glacier
of Rosenlau in Switzerland.
Transporting and abrading Power of Glaciers.—I have described elsewhere
(“Principles” vol. i, chap. xvi, 1867) the manner in which the snow of
the Alpine heights is prevented from accumulating indefinitely in
thickness by the constant descent of a large portion of it by
gravitation. Becoming converted into ice it forms what are termed
glaciers, which glide down the principal valleys. On their surface are
seen mounds of rubbish or large heaps of sand and mud, with angular
fragments of rock which fall from the steep slopes or precipices
bounding the glaciers. When a glacier, thus laden, descends so far as
to reach a region about 3500 feet above the level of the sea, the
warmth of the air is such that it melts rapidly in summer, and all the
mud, sand, and pieces of rock are slowly deposited at its lower end,
forming a confused heap of unstratified rubbish called a _moraine_, and
resembling the _till_ before described (p. 166).
Besides the blocks thus carried down on the top of the glacier, many
fall through fissures in the ice to the bottom, where some of them
become firmly frozen into the mass, and are pushed along the base of
the glacier, abrading, polishing, and grooving the rocky floor below,
as a diamond cuts glass, or as emery-powder polishes steel. The striæ
which are made, and the deep grooves which are scooped out by this
action, are rectilinear and parallel to an extent never seen in those
produced on loose stones or rocks, where shingle is hurried along by a
torrent, or by the waves on a sea-beach. In addition to these polished,
striated, and grooved surfaces of rock, another mark of the former
action of a glacier is the “roche moutonnee.” Projecting eminences of
rock so called have been smoothed and worn into the shape of flattened
domes by the glacier as it passed over them. They have been traced in
the Alps to great heights above the present glaciers, and to great
horizontal distances beyond them.
Alpine Blocks on the Jura.—The moraines, erratics, polished surfaces,
domes, and striæ, above described, are observed in the great valley of
Switzerland, fifty miles broad; and almost everywhere on the Jura, a
chain which lies to the north of this valley. The average height of the
Jura is about one-third that of the Alps, and it is now entirely
destitute of glaciers; yet it presents almost everywhere similar
moraines, and the same polished and grooved surfaces. The erratics,
moreover, which cover it, present a phenomenon which has astonished and
perplexed the geologist for more than half a century. No conclusion can
be more incontestable than that these angular blocks of granite,
gneiss, and other crystalline formations came from the Alps, and that
they have been brought for a distance of fifty miles and upward across
one of the widest and deepest valleys in the world; so that they are
now lodged on a chain composed of limestone and other formations,
altogether distinct from those of the Alps. Their great size and
angularity, after a journey of so many leagues, has justly excited
wonder; for hundreds of them are as large as cottages; and one in
particular, composed of gneiss, celebrated under the name of Pierre à
Bot, rests on the side of a hill about 900 feet above the lake of
Neufchâtel, and is no less than 40 feet in diameter.
In the year 1821, M. Venetz first announced his opinion that the Alpine
glaciers must formerly have extended far beyond their present limits,
and the proofs appealed to by him in confirmation of this doctrine were
acknowledged by all subsequent observers, and greatly strengthened by
new observations and arguments. M. Charpentier supposed that when the
glaciers extended continuously from the Alps to the Jura, the former
mountains were 2000 or 3000 feet higher than at present. Other writers,
on the contrary, conjectured that the whole country had been submerged,
and the moraines and erratic blocks transported on floating icebergs;
but a careful study of the distribution of the travelled masses, and
the total absence of marine shells from the old glacial drift of
Switzerland, have entirely disproved this last hypothesis. In addition
to the many evidences of the action of ice in the northern parts of
Europe which we have already mentioned, there occur here and there in
some of these countries, what are wanting in Switzerland, deposits of
marine fossil shells, which exhibit so arctic a character that they
must have led the geologist to infer the former prevalence of a much
colder climate, even had he not encountered so many accompanying signs
of ice-action. The same marine shells demonstrate the submergence of
large areas in Scandinavia and the British Isles, during the glacial
cold.
A characteristic feature of the deposits under consideration in all
these countries is the occurrence of large erratic blocks, and
sometimes of moraine matter, in situations remote from lofty mountains,
and separated from the nearest points where the parent rocks appear at
the surface by great intervening valleys, or arms of the sea. We also
often observe striæ and furrows, as in Norway, Sweden, and Scotland,
which deviate from the direction which they ought to follow if they had
been connected with the present line of drainage, and they, therefore,
imply the prevalence of a very distinct condition of things at the time
when the cold was most intense. The actual state of North Greenland
seems to afford the best explanation of such abnormal glacial markings.
Greenland Continental Ice.—Greenland is a vast unexplored continent,
buried under one continuous and colossal mass of ice that is always
moving seaward, a very small part of it in an easterly direction, and
all the rest westward, or towards Baffin’s Bay. All the minor ridges
and valleys are levelled and concealed under a general covering of
snow, but here and there some steep mountains protrude abruptly from
the icy slope, and a few superficial lines of stones or moraines are
visible at certain seasons, when no snow has fallen for many months,
and when evaporation, promoted by the wind and sun, has caused much of
the upper snow to disappear. The height of this continent is unknown,
but it must be very great, as the most elevated lands of the outskirts,
which are described as comparatively low, attain altitudes of 4000 to
6000 feet. The icy slope gradually lowers itself towards the outskirts,
and then terminates abruptly in a mass about 2000 feet in thickness,
the great discharge of ice taking place through certain large friths,
which, at their upper ends, are usually about four miles across. Down
these friths the ice is protruded in huge masses, several miles wide,
which continue their course—grating along the rocky bottom like
ordinary glaciers long after they have reached the salt water. When at
last they arrive at parts of Baffin’s Bay deep enough to buoy up
icebergs from 1000 to 1500 feet in vertical thickness, broken masses of
them float off, carrying with them on their surface not only fine mud
and sand but large stones. These fragments of rock are often polished
and scored on one or more sides, and as the ice melts, they drop down
to the bottom of the sea, where large quantities of mud are deposited,
and this muddy bottom is inhabited by many mollusca.
Although the direction of the ice-streams in Greenland may coincide in
the main with that which separate glaciers would take if there were no
more ice than there is now in the Swiss Alps, yet the striation of the
surface of the rocks on an ice-clad continent would, on the whole, vary
considerably in its minor details from that which would be imprinted on
rocks constituting a region of separate glaciers. For where there is a
universal covering of ice there will be a general outward movement from
the higher and more central regions towards the circumference and lower
country, and this movement will be, to a certain extent, independent of
the minor inequalities of hill and valley, when these are all reduced
to one level by the snow. The moving ice may sometimes cross even at
right angles deep narrow ravines, or the crests of buried ridges, on
which last it may afterwards seem strange to detect glacial striæ and
polishing after the liquefaction of the snow and ice has taken place.
Rink mentions that in North Greenland powerful springs of clayey water
escape in winter from under the ice, where it descends to “the
outskirts,” and where, as already stated, it is often 2000 feet thick—a
fact showing how much grinding action is going on upon the surface of
the subjacent rocks. I also learn from Dr. Torell that there are large
areas in the outskirts, now no longer covered with permanent snow or
glaciers, which exhibit on their surface unmistakable signs of ancient
ice-action, so that, vast as is the power now exerted by ice in
Greenland, it must once have operated on a still grander scale. The
land, though now very elevated, may perhaps have been formerly much
higher. It is well-known that the south coast of Greenland, from
latitude 60° to about 70° N., has for the last four centuries been
sinking at the rate of several feet in a century. By this means a
surface of rock, well scored and polished by ice, is now slowly
subsiding beneath the sea, and is becoming strewed over, as the
icebergs melt, with impalpable mud and smoothed and scratched stones.
It is not precisely known how far north this downward movement extends.
Drift carried by Icebergs.—An account was given so long ago as the year
1822, by Scoresby, of icebergs seen by him in the Arctic seas drifting
along in latitudes 69° and 70° N., which rose above the surface from
100 to 200 feet, and some of which measured a mile in circumference.
Many of them were loaded with beds of earth and rock, of such thickness
that the weight was conjectured to be from 50,000 to 100,000 tons. A
similar transportation of rocks is known to be in progress in the
southern hemisphere, where boulders included in ice are far more
frequent than in the north. One of these icebergs was encountered in
1839, in mid-ocean, in the antarctic regions, many hundred miles from
any known land, sailing northward, with a large erratic block firmly
frozen into it. Many of them, carefully measured by the officers of the
French exploring expedition of the Astrolabe, were between 100 and 225
feet high above water, and from two to five miles in length. Captain
d’Urville ascertained one of them which he saw floating in the Southern
Ocean to be 13 miles long and 100 feet high, with walls perfectly
vertical. The submerged portions of such islands must, according to the
weight of ice relatively to sea-water, be from six to eight times more
considerable than the part which is visible, so that when they are once
fairly set in motion, the mechanical force which they might exert
against any obstacle standing in their way would be prodigious.
We learn, therefore, from a study both of the arctic and antarctic
regions, that a great extent of land may be entirely covered throughout
the whole year by snow and ice, from the summits of the loftiest
mountains to the sea-coast, and may yet send down angular erratics to
the ocean. We may also conclude that such land will become in the
course of ages almost everywhere scored and polished like the rocks
which underlie a glacier. The discharge of ice into the surrounding sea
will take place principally through the main valleys, although these
are hidden from our sight. Erratic blocks and moraine matter will be
dispersed somewhat irregularly after reaching the sea, for not only
will prevailing winds and marine currents govern the distribution of
the drift, but the shape of the submerged area will have its influence;
inasmuch as floating ice, laden with stones, will pass freely through
deep water, while it will run a ground where there are reefs and
shallows. Some icebergs in Baffin’s Bay have been seen stranded on a
bottom 1000 or even 1500 feet deep. In the course of ages such a
sea-bed may become densely covered with transported matter, from which
some of the adjoining greater depths may be free. If, as in West
Greenland, the land is slowly sinking, a large extent of the bottom of
the ocean will consist of rock polished and striated by land-ice, and
then overspread by mud and boulders detached from melting bergs.
The mud, sand, and boulders thus let fall in still water must be
exactly like the moraines of terrestrial glaciers, devoid of
stratification and organic remains. But occasionally, on the outer side
of such packs of stranded bergs, the waves and currents may cause the
detached earthy and stony materials to be sorted according to size and
weight before they reach the bottom, and to acquire a stratified
arrangement.
I have already alluded (p. 172) to the large quantity of ice,
containing great blocks of stone, which is sometimes seen floating far
from land, in the southern or Antarctic seas. After the emergence,
therefore, of such a submarine area, the superficial detritus will have
no necessary relation to the hills, valleys, and river-plains over
which it will be scattered. Many a water-shed may intervene between the
starting-point of each erratic or pebble and its final resting-place,
and the only means of discovering the country from which it took its
departure will consist in a careful comparison of its mineral or fossil
contents with those of the parent rocks.
[1] As to the former excess of cold, whether brought about by
modifications in the height and distribution of the land or by altered
astronomical conditions, see Principles, vol. i, (10th ed., 1867),
chaps. xii and xiii, “Vicissitudes of Climate.”
CHAPTER XII.
POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS, concluded.
Glaciation of Scandinavia and Russia. — Glaciation of Scotland. —
Mammoth in Scotch Till. — Marine Shells in Scotch Glacial Drift. —
Their Arctic Character. — Rarity of Organic Remains in Glacial
Deposits. — Contorted Strata in Drift. — Glaciation of Wales, England,
and Ireland. — Marine Shells of Moel Tryfaen. — Erratics near
Chichester. — Glacial Formations of North America. — Many Species of
Testacea and Quadrupeds survived the Glacial Cold. — Connection of the
Predominance of Lakes with Glacial Action. — Action of Ice in
preventing the silting up of Lake-basins. — Absence of Lakes in the
Caucasus. — Equatorial Lakes of Africa.
Glaciation of Scandinavia and Russia.—In large tracts of Norway and
Sweden, where there have been no glaciers in historical times, the
signs of ice-action have been traced as high as 6000 feet above the
level of the sea. These signs consist chiefly of polished and furrowed
rock-surfaces, of moraines and erratic blocks. The direction of the
erratics, like that of the furrows, has usually been conformable to the
course of the principal valleys; but the lines of both sometimes
radiate outward in all directions from the highest land, in a manner
which is only explicable by the hypothesis above alluded to of a
general envelope of continental ice, like that of Greenland (page 170).
Some of the far-transported blocks have been carried from the central
parts of Scandinavia towards the Polar regions; others southward to
Denmark; some south-westward, to the coast of Norfolk in England;
others south-eastward, to Germany, Poland, and Russia.
In the immediate neighbourhood of Upsala, in Sweden, I had observed, in
1834, a ridge of stratified sand and gravel, in the midst of which
occurs a layer of marl, evidently formed originally at the bottom of
the Baltic, by the slow growth of the mussel, cockle, and other marine
shells of living species, intermixed with some proper to fresh water.
The marine shells are all of dwarfish size, like those now inhabiting
the brackish waters of the Baltic; and the marl, in which many of them
are imbedded, is now raised more than 100 feet above the level of the
Gulf of Bothnia. Upon the top of this ridge repose several huge
erratics, consisting of gneiss for the most part unrounded, from nine
to sixteen feet in diameter, and which must have been brought into
their present position since the time when the neighbouring gulf was
already characterised by its peculiar fauna. Here, therefore, we have
proof that the transport of erratics continued to take place, not
merely when the sea was inhabited by the existing testacea, but when
the north of Europe had already assumed that remarkable feature of its
physical geography which separates the Baltic from the North Sea, and
causes the Gulf of Bothnia to have only one-fourth of the saltness
belonging to the ocean. In Denmark, also, recent shells have been found
in stratified beds, closely associated with the boulder clay.
Glaciation of Scotland.—Mr. T. F. Jamieson, in 1858, adduced a great
body of facts to prove that the Grampians once sent down glaciers from
the central regions in all directions towards the sea. “The glacial
grooves,” he observed, “radiate outward from the central heights
towards all points of the compass, though they do not always strictly
conform to the actual shape and contour of the minor valleys and
ridges.”
These facts and other characteristics of the Scotch drift lead us to
the inference that when the glacial cold first set in, Scotland stood
higher above the sea than at present, and was covered for the most part
with snow and ice, as Greenland is now. This sheet of land-ice sliding
down to lower levels, ground down and polished the subjacent rocks,
sweeping off nearly all superficial deposits of older date, and leaving
only till and boulders in their place. To this continental state
succeeded a period of depression and partial submergence. The sea
advanced over the lower lands, and Scotland was converted into an
archipelago, some marine sand with shells being spread over the bottom
of the sea. On this sand a great mass of boulder clay usually quite
devoid of fossils was accumulated. Lastly, the land re-emerged from the
water, and, reaching a level somewhat above its present height, became
connected with the continent of Europe, glaciers being formed once more
in the higher regions, though the ice probably never regained its
former extension.[1] After all these changes, there were some minor
oscillations in the level of the land, on which, although they have had
important geographical consequences, separating Ireland from England,
for example, and England from the Continent, we need not here enlarge.
_Mammoth in Scotch Till._—Almost all remains of the terrestrial fauna
of the Continent which preceded the period of submergence have been
lost; but a few patches of estuarine and fresh-water formations escaped
denudation by submergence. To these belong the peaty clay from which
several mammoths’ tusks and horns of reindeer were obtained at
Kilmaurs, in Ayrshire as long ago as 1816. Mr. Bryce in 1865
ascertained that the fresh-water formation containing these fossils
rests on carboniferous sandstone, and is covered, first by a bed of
marine sand with arctic shells, and then with a great mass of till with
glaciated boulders.[2] Still more recent explorations in the
neighbourhood of Kilmaurs have shown that the fresh-water formation
contains the seed of the pond-weed _Potamogeton_ and the aquatic
Ranunculus; and Mr. Young of the Glasgow Museum washed the mud adhering
to the reindeer horns of Kilmaurs and that which filled the cracks of
the associated elephants’ tusks, and detected in these fossils (which
had been in the Glasgow Museum for half a century) abundance of the
same seeds.
All doubts, therefore, as to the true position of the remains of the
mammoth, a fossil so rare in Scotland, have been set at rest, and it
serves to prove that part of the ancient continent sank beneath the sea
at a period of great cold, as the shells of the overlying sand attest.
The incumbent till or boulder clay is about 40 feet thick, but it often
attains much greater thickness in the same part of Scotland.
Figs. 107-112: Northern shells common in the drift of the Clyde, in
Scotland.
_Marine Shells of Scotch Drift._—The greatest height to which marine
shells have yet been traced in this boulder clay is at Airdie, in
Lanarkshire, ten miles east of Glasgow, 524 feet above the level of the
sea. At that spot they were found imbedded in stratified clays with
till above and below them. There appears no doubt that the overlying
deposit was true glacial till, as some boulders of granite were
observed in it, which must have come from distances of sixty miles at
the least.
Fig. 113: Leda truncata; Fig. 114: Tellina calcarea, Chem.
The shells figured in Figs. 107 to 112 are only a few out of a large
assemblage of living species, which, taken as a whole, bear testimony
to conditions far more arctic than those now prevailing in the Scottish
seas. But a group of marine shells, indicating a still greater excess
of cold, has been brought to light since 1860 by the Reverend Thomas
Brown, from glacial drift or clay on the borders of the estuaries of
the Forth and Tay. This clay occurs at Elie, in Fife, and at Errol, in
Perthshire; and has already afforded about 35 shells, all of living
species, and now inhabitants of arctic regions, such as _Leda truncata,
Tellina proxima_ (see Figs. 113 and 114), _Pecten Grœnlandicus,
Crenella lævigata, Crenella nigra,_ and others, some of them first
brought by Captain Sir E. Parry from the coast of Melville Island,
latitude 76° N. These were all identified in 1863 by Dr. Torell, who
had just returned from a survey of the seas around Spitzbergen, where
he had collected no less than 150 species of mollusca, living chiefly
on a bottom of fine mud derived from the moraines of melting glaciers
which there protrude into the sea. He informed me that the fossil fauna
of this Scotch glacial deposit exhibits not only the species but also
the peculiar varieties of mollusca now characteristic of very high
latitudes. Their large size implies that they formerly enjoyed a
colder, or, what was to them a more genial climate, than that now
prevailing in the latitude where the fossils occur. Marine shells have
also been found in the glacial drift of Caithness and Aberdeenshire at
heights of 250 feet, and in Banff of 350 feet, and stratified drift
continuous with the above ascends to heights of 500 feet. Already 75
species are enumerated from Caithness, and the same number from
Aberdeenshire and Banff, and in both cases all but six are arctic
species.
I formerly suggested that the absence of all signs of organic life in
the Scotch drift might be connected with the severity of the cold, and
also in some places with the depth of the sea during the period of
extreme submergence; but my faith in such an hypothesis has been shaken
by modern investigations, an exuberance of life having been observed
both in arctic and antarctic seas of great depth, and where floating
ice abounds. The difficulty, moreover, of accounting for the entire
dearth of marine shells in till is removed when once we have adopted
the theory of this boulder clay being the product of land-ice. For
glaciers coming down from a continental ice-sheet like that which
covers Greenland may fill friths many hundred feet below the sea-level,
and even invade parts of a bay a thousand feet deep, before they find
water enough to float off their terminal portions in the form of
icebergs. In such a case till without marine shells may first
accumulate, and then, if the climate becomes warmer and the ice melts,
a marine deposit may be superimposed on the till without any change of
level being required.
Another curious phenomenon bearing on this subject was styled by the
late Hugh Miller the “striated pavements” of the boulder clay. Where
portions of the till have been removed by the sea on the shores of the
Forth, or in the interior by railway cuttings, the boulders imbedded in
what remains of the drift are seen to have been all subjected to a
process of abrasion and striation, the striæ and furrows being parallel
and persistent across them all, exactly as if a glacier or iceberg had
passed over them and scored them in a manner similar to that so often
undergone by the solid rocks below the glacial drift. It is possible,
as Mr. Geikie conjectures, that this second striation of the boulders
may be referable to floating ice.[3]
_Contorted Strata in Drift._—In Scotland the till is often covered with
stratified gravel, sand, and clay, the beds of which are sometimes
horizontal and sometimes contorted for a thickness of several feet.
Such contortions are not uncommon in Forfarshire, where I observed
them, among other places, in a vertical cutting made in 1840 near the
left bank of the South Esk, east of the bridge of Cortachie. The
convolutions of the beds of fine and coarse sand, gravel, and loam,
extend through a thickness of no less than 25 feet vertical, or from
_b_ to _c_, Fig. 115, the horizontal stratification being resumed very
abruptly at a short distance, as to the right of _f_, _g._ The
overlying coarse gravel and sand, _ a_, is in some places horizontal,
in others it exhibits cross bedding, and does not partake of the
disturbances which the strata _b_, _c_, have undergone. The underlying
till is exposed for a depth of about 20 feet; and we may infer from
sections in the neighbourhood that it is considerably thicker.
Fig. 15: Section of contorted drift overlying till, seen on left bank
of South Esk, near Cortachie, in 1840.
In some cases I have seen fragments of stratified clays and sands, bent
in like manner, in the middle of a great mass of till. Mr. Trimmer has
suggested, in explanation of such phenomena, the intercalation in the
glacial period of large irregular masses of snow or ice between layers
of sand and gravel. Some of the cliffs near Behring’s Straits, in which
the remains of elephants occur, consist of ice mixed with mud and
stones; and Middendorf describes the occurrence in Siberia of masses of
ice, found at various depths from the surface after digging through
drift. Whenever the intercalation of snow and ice with drift, whether
stratified or unstratified, has taken place, the melting of the ice
will cause such a failure of support as may give rise to flexures, and
sometimes to the most complicated foldings. But in many cases the
strata may have been bent and deranged by the mechanical pressure of an
advancing glacier, or by the sideway thrust of huge islands of ice
running aground against sandbanks; in which case, the position of the
beds forming the foundation of the banks may not be at all disturbed by
the shock.
There are indeed many signs in Scotland of the action of floating ice,
as might have been expected where proofs of submergence in the Glacial
Period are not wanting. Among these are the occurrence of large erratic
blocks, frequently in clusters at or near the tops of hills or ridges,
places which may have formed islets or shallows in the sea where
floating ice would mostly ground and discharge its cargo on melting.
Glaciers or land-ice would, on the contrary, chiefly discharge their
cargoes at the bottom of valleys. Traces of an earlier and independent
glaciation have also been observed in some regions where the striation,
apparently produced by ice proceeding from the north-west, is not
explicable by the radiation of land-ice from a central mountainous
region.[4]
Glaciation of Wales and England.—The mountains of North Wales were
recognised, in 1842, by Dr. Buckland, as having been an independent
centre of the dispersion of erratics—great glaciers, long since
extinct, having radiated from the Snowdonian heights in Carnarvonshire,
through seven principal valleys towards as many points of the compass,
carrying with them large stony fragments, and grooving the subjacent
rocks in as many directions.
Besides this evidence of land-glaciers, Mr. Trimmer had previously, in
1831, detected the signs of a great submergence in Wales in the
Post-pliocene period. He had observed stratified drift, from which he
obtained about a dozen species of marine shells, near the summit of
Moel Tryfaen, a hill 1400 feet high, on the south side of the Menai
Straits. I had an opportunity of examining in the summer of 1863,
together with the Reverend W. S. Symonds, a long and deep cutting made
through this drift by the Alexandra Mining Company in search of slates.
At the top of the hill above-mentioned we saw a stratified mass of
incoherent sand and gravel 35 feet thick, from which no less than 54
species of mollusca, besides three characteristic arctic varieties—in
all 57 forms—have been obtained by Mr. Darbishire. They belong without
exception to species still living in British or more northern seas;
eleven of them being exclusively arctic, four common to the arctic and
British seas, and a large proportion of the remainder having a
northward range, or, if found at all in the southern seas of Britain,
being comparatively less abundant. In the lowest beds of the drift were
large heavy boulders of far-transported rocks, glacially polished and
scratched on more than one side. Underneath the whole we saw the edges
of vertical slates exposed to view, which here, like the rocks in other
parts of Wales, both at greater and less elevations, exhibit beneath
the drift unequivocal marks of prolonged glaciation. The whole deposit
has much the appearance of an accumulation in shallow water or on a
beach, and it probably acquired its thickness during the gradual
subsidence of the coast—an hypothesis which would require us to ascribe
to it a high antiquity, since we must allow time, first for its
sinking, and then for its re-elevation.
The height reached by these fossil shells on Moel Tryfaen is no less
than 1300 feet—a most important fact when we consider how very few
instances we have on record beyond the limits of Wales, whether in
Europe or North America, of marine shells having been found in glacial
drift at half the height above indicated. A marine molluscous fauna,
however, agreeing in character with that of Moel Tryfaen, and
comprising as many species, has been found in drift at Macclesfield and
other places in central England, sometimes reaching an elevation of
1200 feet.
Professor Ramsay[5] estimated the probable amount of submergence during
some part of the glacial period at about 2300 feet; for he was unable
to distinguish the superficial sands and gravel which reached that high
elevation from the drift which, at Moel Tryfaen and at lower points,
contains shells of living species. The evidence of the marine origin of
the highest drift is no doubt inconclusive in the absence of shells, so
great is the resemblance of the gravel and sand of a sea beach and of a
river’s bed, when organic remains are wanting; but, on the other hand,
when we consider the general rarity of shells in drift which we know to
be of marine origin, we cannot suppose that, in the shelly sands of
Moel Tryfaen, we have hit upon the exact uppermost limit of marine
deposition, or, in other words, a precise measure of the submergence of
the land beneath the sea since the glacial period.
We are gradually obtaining proofs of the larger part of England, north
of a line drawn from the mouth of the Thames to the Bristol Channel,
having been under the sea and traversed by floating ice since the
commencement of the glacial epoch. Among recent observations
illustrative of this point, I may allude to the discovery, by Mr. J. F.
Bateman, near Blackpool, in Lancashire, fifty miles from the sea, and
at the height of 568 feet above its level, of till containing rounded
and angular stones and marine shells, such as _Turritella communis,
Purpura lapillus, Cardium edule,_ and others, among which _Trophon
clathratum_ (=_Fusus Bamffius_), though still surviving in North
British seas, indicates a cold climate.
_Erratics near Chichester._—The most southern memorials of ice-action
and of a Post-pliocene fauna in Great Britain is on the coast of the
county of Sussex, about 25 miles west of Brighton, and 15 south of
Chichester. A marine deposit exposed between high and low tide occurs
on both sides of the promontory called Selsea Bill, in which Mr.
Godwin-Austen found thirty-eight species of shells, and the number has
since been raised to seventy.
This assemblage is interesting because on the whole, while all the
species are recent, they have a somewhat more southern aspect than
those of the present British Channel. It is true that about forty of
them range from British to high northern latitudes; but several of
them, as, for example, _Lutraria rugosa_ and _ Pecten polymorphous_,
which are abundant, are not known at present to range farther north
than the coast of Portugal, and seem to indicate a warmer temperature
than now prevails on the coast where we find them fossil. What renders
this curious is the fact that the sandy loam in which they occur is
overlaid by yellow clayey gravel with large erratic blocks which must
have been drifted into their present position by ice when the climate
had become much colder. These transported fragments of granite,
syenite, and greenstone, as well as of Devonian and Silurian rocks, may
have come from the coast of Normandy and Brittany, and are many of them
of such large size that we must suppose them to have been drifted into
their present site by coast-ice. I measured one of granite, at Pagham,
21 feet in circumference. In the gravel of this drift with erratics are
a few littoral shells of living species, indicating an ancient
coast-line.
Glacial Formations of North America.—In the western hemisphere, both in
Canada and as far south as the 40th and even 38th parallel of latitude
in the United States, we meet with a repetition of all the
peculiarities which distinguish the European boulder formation.
Fragments of rock have travelled for great distances, especially from
north to south: the surface of the subjacent rock is smoothed,
striated, and fluted; unstratified mud or _till_ containing boulders is
associated with strata of loam, sand, and clay, usually devoid of
fossils. Where shells are present, they are of species still living in
northern seas, and not a few of them identical with those belonging to
European drift, including most of those already given in Figs. 107 to
112, p. 176. The fauna also of the glacial epoch in North America is
less rich in species than that now inhabiting the adjacent sea, whether
in the Gulf of St. Lawrence, or off the shores of Maine, or in the Bay
of Massachusetts.
The extension on the American continent of the range of erratics during
the Post-pliocene period to lower latitudes than they reached in
Europe, agrees well with the present southward deflection of the
isothermal lines, or rather the lines of equal winter temperature. It
seems that formerly, as now, a more extreme climate and a more abundant
supply of ice prevailed on the western side of the Atlantic. Another
resemblance between the distribution of the drift fossils in Europe and
North America has yet to be pointed out. In Canada and the United
States, as in Europe, the marine shells are generally confined to very
moderate elevations above the sea (between 100 and 700 feet), while the
erratic blocks and the grooved and polished surfaces of rock extend to
elevations of several thousand feet.
I have already mentioned that in Europe several quadrupeds of living,
as well as extinct, species were common to pre-glacial and post-glacial
times. In like manner there is reason to suppose that in North America
much of the ancient mammalian fauna, together with nearly all the
invertebrata, lived through the ages of intense cold. That in the
United States the _Mastodon giganteus_ was very abundant after the
drift period, is evident from the fact that entire skeletons of this
animal are met with in bogs and lacustrine deposits occupying hollows
in the glacial drift. They sometimes occur in the bottom even of small
ponds recently drained by the agriculturist for the sake of the
shell-marl. In 1845 no less than six skeletons of the same species of
Mastodon were found in Warren county, New Jersey, six feet below the
surface, by a farmer who was digging out the rich mud from a small pond
which he had drained. Five of these skeletons were lying together, and
a large part of the bones crumbled to pieces as soon as they were
exposed to the air.
It would be rash, however, to infer from such data that these
quadrupeds were mired in _modern_ times, unless we use that term
strictly in a geological sense. I have shown that there is a fluviatile
deposit in the valley of the Niagara, containing shells of the genera
_Melania, Lymnea, Planorbis, Velvata, Cyclaz, Unio, Helix,_ etc., all
of recent species, from which the bones of the great Mastodon have been
taken in a very perfect state. Yet the whole excavation of the ravine,
for many miles below the Falls, has been slowly effected since that
fluviatile deposit was thrown down. Other extinct animals accompany the
_Mastodon giganteus_ in the post-glacial deposits of the United States,
and this, taken with the fact that so few of the mollusca, even of the
commencement of the cold period, differ from species now living is
important, as refuting the hypothesis, for which some have contended,
that the intensity of the glacial cold annihilated all the species in
temperate and arctic latitudes.
Connection of the Predominance of Lakes with Glacial Action.—It was
first pointed out by Professor Ramsay in 1862, that lakes are
exceedingly numerous in those countries where erratics, striated
blocks, and other signs of ice-action abound; and that they are
comparatively rare in tropical and sub-tropical regions. Generally in
countries where the winter cold is intense, such as Canada,
Scandinavia, and Finland, even the plains and lowlands are thickly
strewn with innumerable ponds and small lakes, together with some
others of a larger size; while in more temperate regions, such as Great
Britain, Central and Southern Europe, the United States, and New
Zealand, lake districts occur in all such mountainous tracts as can be
proved to have been glaciated in times comparatively modern or since
the geographical configuration of the surface bore a considerable
resemblance to that now prevailing. In the same countries, beyond the
glaciated regions, lakes abruptly cease, and in warmer and tropical
countries are either entirely absent, or consist, as in equatorial
Africa, of large sheets of water unaccompanied so far as we yet know by
numerous smaller ponds and tarns.
The southern limits of the lake districts of the Northern Hemisphere
are found at about 40° N. latitude on the American continent, and about
50° in Europe, or where the Alps intervene four degrees farther south.
A large proportion of the smaller lakes are dammed up by barriers of
unstratified drift, having the exact character of the moraines of
glaciers, and are termed by geologists “morainic,” but some of them are
true rock-basins, and would hold water even if all the loose drift now
resting on their margins were removed.