Often these glaciers present a comparatively smooth surface, the pieces of ice of which they are composed varying in size from a pea to a walnut, but not unseldom they are rent by huge fissures, which are impassable by travellers. The most remarkable of these Alpine reservoirs are the glaciers of Mont Blanc, which cover an area of from 90 to 100 square miles. The Mer de Glace, the largest of these, on the northern declivity of the mountain, is 15 miles long, from 3 to 6 miles wide, and from 80 to 120 feet thick. (See Glacier.) The whirlwinds of the Alps are worthy of notice, not only from their terrific violence, often overwhelming the hapless traveller with the blinding snow, but from their frequently setting in motion the dreaded avalanche. So precipitous are many of the slopes of the Alpine peaks, that the giving way of a slight barrier, a tree or bowl-der, perhaps, is sufficient to detach from its original position a vast mass of snow and ice; this, gathering force from its fall, brings sudden and inevitable destruction on whatever may be on its track, burying at times whole villages, crushing extensive forests, and filling up the beds of rivers.
In some parts of the Alps, these masses are so delicately poised that the jar of a footstep, the ringing of a small bell, the breaking of a stick even, is sufficient to cause their precipitation. The optical illusions of the Alps, resulting from a condition of the atmosphere analogous to that of the mirage, have been the subject of much comment. The spectre of the Brocken is the most remarkable of these. It is observed on one of the summits of the Noric Alps. Mont Blanc, the highest mountain in Europe, was first ascended in 1786 by Jacques Balmat, and soon afterward by Dr. Paccard and De Saussure. Its ascent is now a common though dangerous feat of adventurous travellers. - The geological structure of the Alps has long puzzled geologists and given rise to most various and ably supported views. By some, the whole mass composing Mont Blanc and surrounding mountains was considered metamorphic and of comparatively recent date; others regarded the nucleus as primitive and of great antiquity, while the stratified rocks on the lower Alps were referred to different ages from carboniferous to miocene.
The presence of carboniferous plants in formations containing nuramulitic limestone was interpreted by some to indicate that the carboniferous flora survived longer in this region than elsewhere; while others explained the apparent anomaly by an inversion of strata. True granite is rare in the vicinity of Mont Blanc, but occurs in several localities, of which Val-orsine is best known. Both here and at the Col de Balme and the Aiguilles Rouges a por-phyroid granite sends veins into the adjacent gneiss, and appears to be true eruptive granite. Many of the apparently eruptive granites, however, are claimed by M. Alphonse Faure, who has recently (1867) published a valuable work on Alpine geology, as the results of aqueous infiltration. The protogine of the Alps seems to differ from ordinary granite in composition, according to Delesse, in the presence of one or two hundredths of oxide of iron and magnesia. The crystalline protogine forms the centre of Mont Blanc and other peaks, and appears in a curious fan-like form extruded through the secondary strata by breaks which Elie de Beaumont compares to gigantic buttonholes. The flanks are formed by crystalline schists.
Both the protogine and the schists have been considered by all who have studied them stratified rocks, gneissic in structure, passing in places into more schistose varieties, and by no means well separated from each other or the chloritic, talcose, or mica schists of the flanks. The un-erystalline strata in the neighborhood of Mont Blanc includes representatives of the carboniferous, triassic, Jurassic, neocomian, cretaceous, and tertiary. The anthracite system, as held by Scipion Gras, was estimated to have a thickness of 25,000 to 30,000 feet, and included, besides dolomites and gypsum, now referred by Faure to the triassic, layers of anthracite, coal plants, limestones containing belemnites of Jurassic age, gneissic, micaceous, and talcose rooks supposed to be due to the local alteration of members of the anthracite system. To this miscellaneous collection Pillet added in 1860 nummulitic beds. The many recognized disturbances of the strata were made to explain all anomalies of grouping to the satisfaction of some, but Sismonda and Elie de Beaumont, in a memoir presented to the academy of sciences at Paris describing the sections exposed by the Mont Cenis tunnel, hold that there is no evidence of inversion, dislocation, or repetition in the series of 7,000 metres of strata.
Faure indicates the geological history of Mont Blanc, and of the principal portions of the Alps, as follows: In a shallow ocean covering gneiss and crystalline schists, the carboniferous beds were deposited; some disturbance occurred, as secondary deposits are laid down unconformably over all the older formations, gneissic as well as carboniferous; then came the nummulitic limestones and their overlying sandstones, thus embracing a bed from the trias up of a thickness of about 3,800 feet; then came the great upheaval folding these strata, enclosing nummulites and coal plants in crystalline schists; then the work of denudation removed the secondary strata, leaving a few evidences of their former existence, as in the beds more than 100 feet thick of Jurassic and infra-Jurassic age which cap the Aiguilles Rouges. The glacier action on the slopes of the Alps has been studied by De Saus-sure, Agassiz, Forbes, Tyndall, and others, and, from its extent and the comparative accessibility of the Alpine glaciers, has furnished ground for almost all the present knowledge of the geological work of snow and ice on mountain slopes.
The formation of the Alps can no longer be considered an event of recent geological periods, at least so for as the crystalline rocks are concerned, although perhaps the extension and exposure of these ancient crystalline rocks may be recent geologically considered. The phenomenon of transportation of vast blocks of rock across Alpine valleys will be treated of in the article Drift. Many metals are abundantly distributed through the Alpine strata, which will be more particularly described under the head of the different countries in which they occur. - The great height of many of the Alpine summits gives an extraordinary variety to their vegetation. At the base of the mountains it is very rich and beautiful, commingling the productions of a temperate clime with those of a more elevated region, the result of the seeds brought down by the mountain torrents. At the height of 1,600 or 1,700 feet we find a change; the flora is less beautiful, though still rich and abundant; the primula auricula or bear's ear, the gentiana acaulis, the aconitum napellus or wolf's bane, the trollius Europaeus, and the ranunculus aconitifolius, are the most characteristic plants. At 3,300 feet the sol-danella alpina, the crocus vermis, and two species of rhododendrons, adorn the declivities.