It would be impossible within our limits to furnish even a moiety of the details and evidences by which the intricate structure of the Alps has been so successfully unraveled, and the arrangement of nature in the due order of superposition so persistently maintained. We shall simply advert to the equivalents of the English strata which have been satisfactorily ascertained, and shall then consider some of the more interesting phenomena connected with the age, elevation, erratic blocks, and glaciers of this Alpine country.
I. General Structure. The great central axis of the Alpine region, stretching from the Rhone to the Danube, consists mainly of the primary crystalline rocks. The granite is everywhere accompanied by gneiss, mica-schist, chlorite-schist, silicious and serpentine limestones. The upper silurian, devonian, and carboniferous systems are distinctly represented in the eastern Alps; but no traces of the Permian deposits have been detected in them or in any part of southern Europe; while again in following the central parts of the chain from Austria into Switzerland and Savoy, all fossil evidences of the four sedimentary systems disappear. The conclusion arrived at by geologists, therefore, is that for these palæozoic and triassic formations there exist no representatives among any of the vast piles of strata of the western Alps; or, if they ever had a place in this part of the chain, that they have been obliterated by the powerful transmuting action of metamorphism, or plunged to inaccessible depths beneath the upraised edges of the primary series. Coal plants, and anthracite coal itself, have both indeed been found in the valley of the Arve in Savoy, at Tarentaise, Maurienne, and along the base of Mont Blanc; but as they occur in connection with belemnites, these beds have been referred by M. E. de Beaumont and others to the Lias formation, which is clearly determined by its numerous animal fossils to exist in this part of the chain. The remarkable picturesque rocks of Varennes, Duron, and the Col de Balme belong to the lias deposit—the grandest specimens, perhaps, of natural architecture anywhere to be seen.
But, making allowance for all the uncertainties of their lithological complement, and quitting all points of a doubtful character, it has been satisfactorily established that the flanks of the Swiss Alps are covered by a series of sedimentary deposits of vast thickness, which form a true transition from the newer secondary into the older tertiary strata. The normal arrangement of rocks within these limits is complete, beginning with the lias and terminating in distinctly recognized beds of the tertiary pleiocene group.
The lias formation is largely developed along the northern, eastern, and south-western side of the chain, forming an immense belt from near the foot of the Jungfrau, in the central district of Switzerland, to Savona in the Gulf of Genoa. The oolitic formation succeeds, on a scale of still greater magnitude, having a continuous stretch from the Mediterranean at Toulon to Vienna, on the Danube; again constituting an enormous deposit along the Jura range of mountains; and then by Ulm, Altmuhl, and Amberg, to Beyruth, with its celebrated bone caves, in the heart of Germany. The Oxfordian group are represented by the “Neocomian” limestones, a series of hard subcrystalline strata, abounding in fossils of the gault and upper green-sand. To these succeed beds of red, gray, and white marly limestones, containing Gryphææ, Inocerami, and Ananchytes, and regarded as the equivalents, as they are undoubtedly in the position, of the white chalks of England. A supercretaceous group, consisting of nummulitic and shelly rocks, the “flysch” of the Swiss, constitutes the close of the secondary, and graduates conformably and insensibly upward, by mineral and zoological passages, into the eocene system. The vast beds of strata, which are termed the “molasse” and “nagelflue,” contain in the lower series a large proportion of living species of marine shells, while the associated and overlaying strata of terrestrial origin are loaded with forms all of which are extinct. In this group there is nevertheless shadowed forth a type of rocks characteristic both of a miocene and pleiocene age; but so anomalous is their arrangement, that the younger are often found to dip under the older rocks out of which they have been formed. And as of these, so generally of the entire Alpine series now referred to, the position of the various groups in particular localities can only be unraveled in their flexures, dislocations, and displacements, by means of the organic remains with which they severally abound.
II. The Superficial Accumulations embrace a wide-spread class of geological phenomena. These have originated in causes some of which are still in active operation, others are dormant, and others again may be considered as belonging to agencies which may be termed extraordinary, or permitted only at intervals to display themselves. The effects of their operations are visible, less or more, in every part of the surface of the globe. They have been termed the Pleistocene group, and consist of both marine and fresh water materials. To these are referred the bowlder-clay formation, the vast deposits of sand and gravel heaped up in valleys, the erratic blocks spread over hill-tops, and the various kinds of detrital matter which, although often laminated, is loose and unstratified, and clearly distinguishable from the more indurated and subjacent beds composing the earth’s crust. Nor in gathering up the links of this extended field of review, will it be possible to omit all mention of glaciers and their moraines, so intimately connected with Alpine scenery.
The sand, gravel, and drift accumulations of every kind are common to every country where waters flow or valleys exist. They cover the great straths of Scotland, the low steppes of Russia, the lofty gorges of the Himalaya, the desert wastes of Africa, and the elevated plateaux of North and South America. Among the Rocky Mountains they are of the most varied character, and are spread over extensive areas in those sterile regions, high up among the sources of the great American rivers. Wherever a stream falls into another stream, a stream into a lake, a lake into a river, or a river into the sea, bars, gravelly shoals, and deltas are found to exist, or to be in the act of formation. Accumulations of this class, therefore, are to be regarded as of various periods, as they are evidently the results of causes of continual operation, ordinary as well as extraordinary. Many of such phenomena, however, are as clearly the indication of a state of things which no longer exists. Whether by a subsidence in the sea-bottom, or an elevation of the land, they are now raised far above the influence of the element within which they were collected, and to whose abrading powers they owe their laminated structure. Such, in particular, are those regular-shaped terraces as well as detached hillocks of sand and gravel, several hundred feet in depth, so common in the straths of Scotland and valleys of Switzerland, through which arms of the sea or of great inland lakes once penetrated, and over whose shores and bottoms the debris of the mountains gradually accumulated. “The Sea Margins,” the work of the accomplished Robert Chambers, contains a minute and interesting detail of these facts, gleaned from varied sources of reading and most extensive personal observation, and clearly warranting the inference that the sea at no very remote period covered vast districts of country from which it has now receded.
The Bowlder Clay immediately underlies the gravelly beds which have been noticed. Betwixt the two classes of drift there is a clear line of demarkation, although both sand and gravel are often in considerable masses included in the plastic mud which chiefly characterizes the bowlder clay. This formation is of great extent, covering the whole of the north of Europe, a large portion of northern Asia, and in America extending from the Arctic Sea to Boston; massed up in every ravine, and ranging from the lowest valleys to two thousand feet on the mountain slopes, where it is often accumulated to a great depth. One striking peculiarity of the bowlder clay is, that huge blocks of stones of all ages are imbedded in the mass in every region and country where it is found: hence the name. The bowlders are not always of local origin; on the contrary, the parent rock is more generally situated at remote distances, even from five to eight hundred miles. Thus the Scottish Grampians furnish the greater proportion of the huge blocks which are scattered over the lowland and midland counties of Scotland. The Lammermuirs, the Cheviots, the Lake Mountains of Cumberland, have strewed their wreck over the vales of the Tweed and Northumberland, through Yorkshire and the midland plains of England. The chalks of Denmark and Norway are spread out on every shoal and bank in the German Ocean to the British shores; while again, through all Friesland and central Germany, the primary rocks of Scandinavia are as distinctly to be traced. The erratic-block family have in like manner traveled over France, those of Britanny and Normandy penetrating to the basin of the Loire; the Cantal down even to the shores of the Mediterranean. Switzerland, perhaps, contains the most interesting specimens of this universal drift-wreck; as, on the sides of the Jura, at an elevation of four thousand feet, at Monthey, where they give a feature to the landscape, and on the east bank of the lake of Geneva, lies the celebrated Pierre de Gouté, which figures in the Huttonian controversy, measuring about ten feet in height by fifteen to twenty in breadth and length. Mont Blanc is conjectured to have been the source of most of these pierres roulés, which have been transported across the valley of the Rhone, or lifted sheer over the mountains of Savoy, and are now at the distance of sixty and seventy miles, lying in all the passes and ridges of the Jura.
Various explanations have been given of the origin and deposition of the bowlder-clay formation, as well as of the erratic block-drift, for the two can scarcely be separated in the question of cause and effect. The bowlders, for example, are sometimes in the mass of clay itself, sometimes they are lying loose on the surface, in many instances they are spread over areas where no clay exists; but in most cases maintaining their unmistakable character of being water-worn, rounded, and covered with striæ. Both classes of phenomena, therefore, are supposed to be referable to the same period of time, as they probably have originated in the same series of causes. One theory advanced in explanation of both, is the agency of powerful currents that swept over Britain and the adjacent continents, generally in a north and north-westerly direction, bearing along with them soil, gravel, and the larger debris of rocks; and as obstructions occurred, or the violence of the currents subsided, the heterogenous materials were deposited in the various countries and at the different elevations in which they are found. The direction of the currents, often from different centers, is indicated clearly by the position and lithology of the mountains from which the blocks have been transported, no less than by the fact that the greatest accumulations of drift and bowlders are to be observed at the south-eastern extremities of such gorges and valleys as were open to the diluvial action. But the hypothesis fails in giving a satisfactory account of the transport of the larger blocks, often of sixty to a hundred tons weight, over a course of many hundreds of miles, plunging through hollows, and now stranded on mountain slopes several thousand feet above. The theory of icebergs, as the transporting agency, meets this difficulty; and accordingly, in one form or other, such a cause or agent, of widely-prevailing influence, is almost universally adopted into the creed of geologists. This theory implies, that those portions of Europe now covered with the bowlder-clay formation were submerged after the deposition and consolidation of the tertiary strata—that this submergence was the result of a change in the earth’s axis or some extraordinary alteration in its planetary relations—that a great arctic glacial continent subsided and disappeared beneath the waters—and that vast floating masses of ice, inclosing rock loosened from the sinking land, penetrated southward, grazing and polishing the harder substances that lay in their course, or carrying along with them the more yielding and transportable materials. Admit all or even a limited number of such assumptions, and we know from what is occurring in recent times, that the cause is quite adequate to the production of the effect. Sir James Ross, in his late humane exploratory expedition, encountered in the polar regions icebergs from a hundred to three hundred feet in height, and from a quarter to half a mile in length. Two-thirds of every iceberg float beneath the water. What a carrier power, at once for erosion and transport, in every one of these frozen floating mountains! The Polar ocean still maintains its great southward current to the equatorial seas, modified by the headlands and inequalities of bottom which occur in its progress; and then, as now, the icebergs driven along this highway of waters would drop, at intervals, portions of their stony load, to take up at other stations whatever was prepared to adhere to them. Hence the difficulty vanishes as to the large detached blocks so often found on the elevated sides of mountains. Hence, too, the explanation of those collected groups which are entirely free from any admixture of clay. And hence, upon the retreat of the waters and the elevation of the land, it is reasonable to infer that many districts would be swept bare again of their mud, while the bowlders would remain, and that in other quarters ridges and the deeper accumulations would be formed. “Both theories,” however, as stated by Mr. Page in his excellent treatise, “Rudiments of Geology,” “are beset with difficulties; and though the latter accounts more satisfactorily for most of the phenomena of the erratic block group, still there are many points respecting the distribution and extent of the deposit to be investigated before either can be finally adopted. All that can be affirmed in the present state of the science is the composition and nature of the clay, gravel, and bowlders—the course of the currents concerned in their deposition—the fact of the land having a configuration of hill and valley, not differing much from what now exists—and the peculiar scantiness, if not total absence, of organic remains.”
Whether this mysterious cataclysm occurred before or within the modern epoch is a question which, as yet, has not by any means been determined. The few organic remains detected in the deposit are of marine origin—one or two species of shells—but all identical with species now existing. The presumption is that the climate which prevailed over these northern regions during the period was extremely low. But how long it lasted, and why there are no types preserved, in all that congeries of materials of the terrestrial fauna and flora of the period, are points both of them of a very perplexing kind. Whether just dawning upon the advent of man, or within the actual era of his history, certain it is that these are the results of a chaotic condition over a large portion of our planet, of which, if we except the deluge, we have no record nor memorials in any of the after changes and modifications of its surface. Shall we add, as indicative of a final cause appearing in and overruling the tumultuous agitation, that to this source is to be traced great part of the soil which covers the valleys and mountain sides of all the submerged districts? that hereby extensive lakes were silted up, the flinty rock concealed by fertile earth, and the steep acclivity made accessible to the husbandman? One thing is clear, that all the latest tertiary strata in this alpine region have, after their consolidation, been disturbed and broken up: it is upon their inverted edges that the superficial accumulations have been deposited and now rest: and whether the submergence of Europe, and other parts of the globe, was simultaneous or not with the cause of their movement and overthrow, a superintending wisdom and purpose are unquestionably discernible in those accessions of soil and other economic arrangements that resulted from the change.
There is another theory, however, which has been applied to the explanation of these phenomena—namely, the theory of glaciers, as illustrated in the works of Venetz, Charpentier, Agassiz, and Forbes.