334
ALPS
along their outcrop, and thus the broader and more level. north is represented by the Pennines ? The fact that the zone These are generally excavated in a mass softer than those of the Briangonnais (an important trough of sedimentary adjacent on either side, whether it is intercalated or rocks) in the main keeps to the west of the Grande Casseinfolded. When this mass is prolonged for a great dis- Pourri massif seems to link the latter to the central system. tance the strike valleys are sometimes divided by low It is generally admitted that, whatever be the case in watersheds into different drainage systems. Of this, the regard to Mont Blanc, the headwaters of streams flowing Upper Rhone, the Reuss at its outset, and the Yorder towards Italy have frequently trespassed on those Movement Rhein afford an excellent example. They are excavated in running in the opposite direction. The most ofth?60 an infold, composed mainly of Jurassic rocks, which is striking instance is afforded by the Maloja larger divided by watersheds at the Furka and Oberalp passes. Pass. Here the head of the Inn valley is an waferAs valleys of strike are both supplied and drained by almost level trough, about 9 miles long, be- shedsvalleys of dip, the course of an Alpine river, unless tween lofty mountains, which is terminated at the actual affected by some of the irregularities mentioned above, has pass by precipices descending to the Yal Bregaglia. But a rude resemblance to the outline of a flight of steps. it is inferred from studying the surrounding district that In the Eastern Alps, and in the southern more than in the watershed once lay some 6 miles farther south, the the northern region, the main lines of drainage run Yal Forno and Yal Albigna being then tributaries of the towards the east. The precise cause is not Inn, but that it has been completely cut away by the eas clulses. ily determined, but probably it is connected greater erosive action of the Maira, which has annexed the with the rather abrupt cessation of the chain at drainage of these two glens. Those shorter trenches, this extremity. The watershed then follows the central or which afford a passage to the great Alpine roads, may crystalline range until on approaching the upper part of be similarly explained, and such apparent anomalies as the Inn valley it bends towards the south, returning from the structure of the mountains near Zermatt. Here the the Maloja Pass, in the neighbourhood of the Rheinwald- truncation of the range between the two arms of the Yisp horn, nearly to its former position. Thus the upper waters at the Strahlhorn, the wide gap between it and Monte of the Inn and the Hinter Rhein are supplied from a dis- Rosa, and the apparent disappearance of the Gornergrattrict distinctly south of the usual line of the watershed. Stockhorn spur, may be due to the recession of the head The precise cause of this flexure is also difficult to deter- of the Yal Anzasca. mine, but it is probably connected with the syncline, The Alpine chain, as it exists, is the result of two mentioned in a former paragraph. The watershed after- separate sets of movements during the Tertiary era in a wards follows the crest of the Lepontine and Pennine region which, as stated above, had already been Alps, having to the north a second range—largely con- the scene of disturbances. But the history Relation of sisting of crystalline rocks—the Bernese Oberland—higher of the former set is not yet certain. Some ^J^0ve' than the former and not much inferior to the latter, through authorities think that the apparent change in gashes in which the Reuss and the Rhone discharge their general structure, near the headwaters of the Inn, indiwaters. From this we infer that the structure of this cates that the Alps to the east and the west were proregion, when the river system of the Alps was first deter- duced by independent disturbances. But even if the mined, was a simpler one, more like that of the Eastern other view be adopted, the movements may not have been Alps; the Pennine Alps, forming a watershed, flanked on uniform or quite simultaneous. The effects of the either side by sedimentary ranges, of which the southern second set were, at any rate, more marked in the has been xemoved by denudation, and the northern greatly Central and Western Alps, as is shown by the facts already exaggerated by a later series of movements, which, how- mentioned, and by the upheaval of the nagelflue (gravel ever, must have been slow enough to allow the rivers to from the rivers of the Oligocene Alps), to full 6000 feet deepen their valleys pari passu with the rise. Though above sea-level in the Rigi and the Speer. Some recent the range of Mont Blanc is connected with that of the writers have maintained that parts of the Alps have been Oberland, its crest forms the watershed, for this passes from affected by torsional movements. It is, however, difficult the Great St Bernard over the low gap of the Col Ferret, to understand the mechanical conditions under which such and by the Little St Bernard returns to its former course movements, in the strict sense of the term, could be proat the Rutor; a diversion probably due to the excep- duced in the earth’s crust, though no doubt unequal tional elevation of the northern range in the vicinity of resistances to pressure and the joint effects of successive Mont Blanc, which has enabled it to discharge part of its sets of thrusts, acting in different directions, would present water into the Yal d’Aosta. Its original inferiority is a very similar aspect. Apart, however, from this it has however, suggested by the fact that one tributary of the been demonstrated, especially since 1870, that the strucDurance actually drains the north-eastern flank of the now- ture of the Alps is far more complicated than was formerly dommant massif. _ South of the Little St Bernard Pass the watershed keeps in its old position along the crest of the supposed. It was also believed that in the Alps igneous of Tertiary age occurred only in the Vicentin and in crystalline mass which encloses the Italian plain, while rocks one or two isolated spots; recently, however, it has been the Prolongation of the Mont Blanc massif may be traced tin ough the Tarentaise and Maurienne as far as the High maintained (though the question is still sub judice) that Alps of Dauphine. The structure of these masses, how- some of the intrusions in the Dolomite district belong to ever is anything but simple. The Romanche, rising on the this era instead of to the Trias. Much attention has been given to the movements and Col de Lautaret, forces its way through two crystalline physics of glaciers. In the Alps their average motion is ranges, that of the Grandes Rousses and of the Belledonne between which is an infold of Jurassic rocks Do these not far from a foot a day; while the study of other regions two crystalline masses, together with the other and larger has shown that, cceteris paribus, the larger the GIac!ers one, represent the double range system (Mont Blanc and the glacier the quicker its pace. Special attention has been paid to the changes in volume exhibited by the Brevent) farther north; or should the chief Dauphine mass (that around the Pomte des Ecnns) be connected, as some Alpine glaciers. These are sometimes irregular, but as a think, with the crystalline ridge, of which the Grande rule they affect the majority of glaciers almost simultaneCasse and Mont Pourri form parts; or is this latter ridge ously. A universal shrinkage set in soon after 1860, but merely an outlier of the great central system, which farther now the tide seems to have turned. The diminution in some cases has been very great; for instance, the thickness of
