CHAP. XV.

PHASES OF ALPINE GLACIAL ACTION.

321

4th. A second retreat of the glaciers took place when they gradually shrank nearly into their present limits, accompanied by another accumulation of stratified gravels, which form in many places a series of terraces above the level of the alluvial plains of the existing rivers.

In the gorge of the Dranse, near Thonon, M. Morlot discovered no less than three of these glacial formations in direct superposition, namely, at the bottom of the section, a mass of compact till or boulder-clay (No. 1) twelve feet thick, including striated boulders of Alpine limestone, and covered by regularly stratified ancient alluvium (No. 2) 150 feet thick, made up of rounded pebbles in horizontal beds. This mass is in its turn overlaid by a second formation (No. 3) of unstratified boulder clay, with erratic blocks and striated pebbles, which constituted the left lateral moraine of the great glacier of the Rhone, when it advanced for the second time to the lake of Geneva. At a short distance from the above section, terraces (No. 4) composed of stratified alluvium are seen at the heights of 20, 50, 100, and 150 feet above the lake of Geneva, which, by their position, can be shown to be posterior in date to the upper boulder-clay, and therefore belong to the fourth period, or that of the last retreat of the great glaciers. In the deposits of this fourth period, the remains of the mammoth have been discovered, as at Morges, for example, on the lake of Geneva. The conical delta of the Tinière, mentioned at p. 27 as containing at different depths monuments of the Roman as well as of the antecedent bronze and stone ages, is the work of alluvial deposition going on when the terrace of 50 feet was in progress. This modern delta is supposed by M. Morlot to have required 10,000 years for its accumulation. At the height of 150 feet above the lake, following up the course of the same torrent, we came to a more ancient delta, about ten times as large, which is therefore supposed to be the monument of about ten times as

322

SUCCESSION OF GLACIAL DEPOSITS.

CHAP. XV.

many centuries, or 100,000 years, all referable to the fourth period mentioned in the preceding page, or that which followed the last retreat of the great glaciers.*

If the lower flattened cone of Tinière be referred in great part to the age of the oldest lake-dwellings, the higher one might, perhaps, correspond with the post-pliocene period of St. Acheul, or the era when Man and the Elephas primigenius flourished together; but no human remains or works of art have as yet been found in deposits of this age, or in any alluvium containing the bones of extinct mammalia in Switzerland.

Upon the whole, it is impossible not to be struck with an apparent correspondence in the succession of events of the glacial period of Switzerland, and that of the British Isles before described. The time of the first Alpine glaciers of colossal dimensions, when that chain perhaps was several thousand feet higher than now, may have agreed with the first continental period alluded to at pp. 241 and 282, when Scotland was invested with a universal crust of ice. The retreat of the first Alpine glaciers, caused partly by a lowering of that chain, may have been synchronous with the period of great submergence and floating ice in England. The second advance of the glaciers may have coincided in date with the re-elevation of the Alps, as well as of the Scotch and Welsh mountains; and lastly, the final retreat of the Swiss and Italian glaciers may have taken place when Man and the extinct mammalia were colonising the north-west of Europe, and beginning to inhabit areas which had formed the bed of the glacial sea during the era of chief submergence.

But it must be confessed, that in the present state of our knowledge, these attempts to compare the chronological relations of the periods of upheaval and subsidence of areas so

Morlot, Terrain quaternaire du Bassin de Léman, Bulletin de Société

Vaudoise des Sciences Naturelles, No. 44.

CHAP. XV.

COLD PERIOD IN SICILY AND SYRIA.

323

widely separated as are the mountains of Scandinavia, the British Isles, and the Alps, or the times of the advance and retreat of glaciers in those several regions, and the greater or less intensity of cold, must be looked upon as very conjectural.

We may presume with more confidence that when the Alps were highest and the Alpine glaciers most developed, filling all the great lakes of northern Italy, and loading the plains of Piedmont and Lombardy with ice, the waters of the Mediterranean were chilled and of a lower average temperature than now. Such a period of refrigeration is required by the conchologist to account for the prevalence of northern shells. in the Sicilian seas about the close of the newer pliocene or commencement of the post-pliocene period. For such shells as Cyprina islandica, Panopoa Norvegica, (= P. Bivona Philippi), Leda pygmæa, and some others, enumerated among the fossils of the latest tertiary formations of Sicily by Philippi and Edward Forbes, point unequivocally to a former more severe climate. Dr. Hooker also, in his late journey to Syria (in the autumn of 1860), found the moraines of extinct glaciers, on which the whole of the ancient cedars of Lebanon grow, to descend 4,000 feet below the summit of that chain. The temperature of Syria is now so much milder, that there is no longer perpetual snow even on the summit of Lebanon, the height of which was ascertained to be 10,200 feet above the Mediterranean.

Such monuments of a cold climate in latitudes so far south as Syria and the north of Sicily, between 33° and 38° north, may be confidently referred to an early part of the glacier period, or to times long anterior to those of Man and the extinct mammalia of Abbeville and Amiens.

* Hooker, Natural History Review, No. 5, January 1862, p. 11.

324

NATURE AND ORIGIN OF THE LOESS.

CHAP. XVI.

CHAPTER XVI.

HUMAN REMAINS IN THE LOESS, AND THEIR PROBABLE AGE.

NATURE, ORIGIN, AND AGE OF THE LOESS OF THE RHINE AND DANUBE IMPALPABLE MUD PRODUCED BY THE GRINDING ACTION OF GLACIERS-DISPERSION OF THIS MUD AT THE PERIOD OF THE RETREAT OF THE GREAT ALPINE GLACIERS CONTINUITY OF THE LOESS FROM SWITZERLAND TO THE LOW COUNTRIES — CHARACTERISTIC ORGANIC REMAINS NOT LACUSTRINE ALPINE GRAVEL IN THE VALLEY OF THE RHINE COVERED BY LOESS GEOGRAPHICAL DISTRIBUTION OF THE LOESS AND ITS HEIGHT ABOVE THE SEA - FOSSIL MAMMALIA-LOESS OF THE DANUBE OSCILLATIONS IN THE LEVEL OF THE ALPS AND LOWER COUNTRY REQUIRED TO EXPLAIN THE FORMATION AND DENUDATION OF THE LOESS-MORE RAPID MOVEMENT OF THE INLAND COUNTRY — THE SAME DEPRESSION AND UPHEAVAL MIGHT ACCOUNT FOR THE ADVANCE AND RETREAT OF THE ALPINE GLACIERS- HIMALAYAN MUD OF THE PLAINS OF THE GANGES COMPARED TO EUROPEAN LOESSHUMAN REMAINS IN LOESS NEAR MAESTRICHT, AND THEIR PROBABLE ANTIQUITY.

INTER

Nature and Origin of the Loess.

NTIMATELY connected with the subjects treated of in the last chapter, is the nature, origin, and age of certain loamy deposits, commonly called loess, which form a marked feature in the superficial deposits of the basins of the Rhine, Danube, and some other large rivers draining the Alps, and which extend down the Rhine into the Low Countries, and were once perhaps continuous with others of like composition in the north of France.

It has been reported of late years that human remains have been detected at several points in the loess of the Meuse around and below Maestricht. I have visited the localities referred to; but, before giving an account of them, it will be desirable to explain what is meant by the

CHAP. XVI.

MUD PRODUCED BY GLACIERS.

325

loess, a step the more necessary, as a French geologist, for whose knowledge and judgment I have great respect, tells me he has come to the conclusion that 'the loess' is 'a myth,' having no real existence in a geological sense, or as holding a definite place in the chronological series.

*

No doubt it is true that in every country, and at all geological periods, rivers have been depositing fine loam on their inundated plains in the manner explained above at p. 34, where the Nile mud was spoken of. This mud of the plains of Egypt, according to Professor Bischoff's chemical analysis, agrees closely in composition with the loess of the Rhine. I have also shown (p. 201), when speaking of the fossil man of Natchez, how identical in mineral character, and in the genera of its terrestrial and amphibious shells, is the ancient fluviatile loam of the Mississippi with the loess of the Rhine. But granting that loam presenting the same aspect has originated at different times and in distinct hydrographical basins, it is nevertheless true that, during the glacial period, the Alps were a great centre of dispersion, not only of erratics, as we have seen in the last chapter, and of gravel, which was carried farther than the erratics, but also of very fine mud, which was transported to still greater distances and in greater volume down the principal river-courses between the mountains and the sea.

Mud produced by Glaciers.

They who have visited Switzerland are aware that every torrent which issues from an icy cavern at the extremity of a glacier is densely charged with an impalpable powder, produced by the grinding action to which the subjacent floor of rock and the stones and sand frozen into the ice are exposed in the manner before described. We may therefore readily

* Chemical and Physical Geology, vol. i. p. 132.