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bottoms. While some geologists have attributed both to the action of glacier-ice, by others they are believed to be the result of aqueous erosion. The cirques themselves are doubtless in many cases the work of converging torrents, aided by frost. Very frequently, however, frost, rather than running water, has been the chief eroding agent, as may be seen in Norway, where, in immediate proximity to the ndvd-\\ne, cirques are now being formed. The basin at the bottom of a cirque, however, is the work neither of running water nor of frost alone, but has been ground out by glacier-ice. In the Highlands and the Southern Uplands of Scotland the head-waters of streams and rivers often proceed from cirque-basins, especially in the more elevated districts. Many of the smaller feeders, however, come from cirques which have no basin, and this is particularly the case in the less elevated portions of the mountain regions. The origin of the latter is obvious; we see them being formed at present. Springs, summer torrents, snow-water, and frost—all play their parts. The converging mountain-slopes direct the drainage to one point, the result being the formation of a more or less abrupt funnel-shaped depression resembling the section of an inverted hollow cone. The formation of a basin at the apex of this inverted cone by aqueous action is impossible. The torrent escaping from the cirque simply digs its channel deeper, cuts its way back, and by its undermining action tends to increase the slope of the surrounding walls. Add to this the action of frost in splitting up the rocks and
detaching larger and smaller masses, and one can readily understand how a cirque must increase in extent. Cirques of this character occur under all conditions of climate and in every mountain region of suitable structure, in temperate, subtropical, and tropical zones alike.1 But the flat-bottomed cirque is restricted to regions which are now, or have recently been, subjected to glaciation. Cirque-basins are familiar features in the Alpine lands of temperate latitudes, and they are met with likewise, but only at lofty elevations, in the warmer zones. When a mountain area was subjected to glaciation, the cirques, which occurred in immediate proximity to the snowline, would form admirable reservoirs for the accumulation of snow and tu<z■d, and the formation of " summit glaciers." The shape of a cirque would greatly favour glacial erosion by enabling the ice to concentrate its grinding and disrupting action upon the point towards which the mountain-slopes converged. Hence, in time, the bottom of such a cirque could not fail to be ground out, and the basin thus formed, owing to the conditions that so specially favoured erosion,
1 Although the true cirque usually presents the appearance of a niche-like indentation in a mountain-slope, not a few valleys terminate upwards in great amphitheatre-like cirques, the walls of which are often very steep. Such cirque-valleys appear now and again in our European mountains. As examples, may be cited the great cirque of Gavarni in the Pyrenees, the valleys of the Hallstadter See and the Konigs See, and of the Trenta and the Wochein in the Alps, and the great cirque-valleys of Norway, such as that near Lunde (Jostedalsbrae), the precipitous encircling walls of which rise more than 3000 feet above the bottom of the valley. Glen Eunach (Cairngorm Mountains) is a good example of a Scottish valley with a cirque-shaped head. Such great cirque-valleys often contain lakes.
would tend to be relatively deeper than the rockbasins excavated in a broad mountain-valley.
The vertical distribution of corrie-basins in any ^jiven tract of mountains shows that they are closely related to former snow-lines. They occur in belts, or zones, and are not irregularly scattered over a whole region. Amongst the Scottish mountains two such zones can be recognised. In the lower part of these the corrie-basins range from 1500 feet to 2400 feet or thereabouts; in the upper they occur between 2400 feet and 3400 feet. Consequently, the two zones are met with together only among the most elevated mountain-groups. In the mountains of Middle Germany the zone of cirque-basins lies between 3000 feet and 3500 feet above sea-level; and Professor Partsch has pointed out the significant fact that the cirques, as we follow them from west to east, rise to higher and higher levels, showing, as he says, that the snowline of glacial times gradually ascended as it passed eastward into the interior of the continent. Similarly in the Alps and the Pyrenees, cirque-basins occur in definite zones, and form harmonious systems in the several mountain-groups, each zone marking out a former snow- or ndvMevel.1
1 Professor Penck gives the following table to show the relative heights attained by mountain-lakes—the zones of greatest development of high-level lakes. He includes in this table not only cirque-lakes, but many small barrierlakes:
Norway ..... 1000-1600 metres.
Hohe Tatra ..... 1500-2100"
GraubUnden Alps .... 2000-2700"
It is interesting further to note that in North and Middle Europe the cirque-basins affect chiefly the mountain-slopes that face the north and north-east. Thus of 78 in the uplands of Norway, according to Helland, 50 face the north, while 19 open towards the east. So, again, Partsch states that of 35 in the mountains of Middle Germany 19 look north and north-east, 13 east and south-east, and only 3 face the south and west. This distribution, as Penck remarks, is quite in keeping with existing conditions, for at present most snow accumulates on northern and eastern exposures. On southern exposures it quickly melts, while from the western declivities of the mountains it is blown away by the prevailing west winds.
2. Mountain-Valley Basins. This class includes all lakes of glacial origin occurring in mountain-valleys or closely connected with these. In some regions they are seen only at the very heads of the valleys, which may be cirque-shaped or not; elsewhere they appear towards the lower ends of the valleys, from which they now and again extend into the low grounds; or they may occur outside of the mountains altogether, opposite the mouths of great mountain
valleys. Many of these are rock-basins, others are barrier-basins, that is, the water has been impounded by the unequal deposition of glacial and fluvio-glacial detritus. The large majority, however, partake of both characters; the lakes occupy rock-basins, the lower ends of which have been heightened by morainic and fluviatile accumulations. Many of the lakes in question attain a great depth. Amongst the lakes of the Alps, for example, we find depths of 469 feet (Zurich), 826 feet (Constance), 1013 feet (Geneva), 1135 feet (Garda), 1341 feet (Como), 2800 feet (Maggiore). Similar relatively deep lakes occur in Scotland. Loch Lomond, for instance, has an extreme depth of 630 feet, and Loch Ness of 780 feet.