242 NATURAL SCIENCE. April, 



which, in the case of each hill, coincide with fault-planes. Again and 

 again, among the Lake District fells, hills may be seen which have 

 been carved by stream-erosion out of blocks of rock, whose size is 

 determined by planes [of weakness of varying character. Some of 

 these planes are faults, often converted into mineral veins, others 

 master-joints, and others, again, dykes formed of material more easily 

 eroded than the surrounding rocks. Mickledore is an example of the 

 last. Clifton Ward announced some years ago that it owed its 

 existence to a dyke : if that dyke had not been intruded along the 

 line of Mickledore, Scawfell Pike and Scawfell would have formed 

 one hill, and the great cliff of Scawfell, which has become the favourite 

 climbing-ground of an increasing number of cragsmen, would have 

 had no existence. In some cases we see this process of fission com- 

 mencing. On the north-west side of the Gables the stream which 

 forms the head-waters of the river Liza, occupying Ennerdale, runs 

 along a line of fault ; and on the south-east side the stream down 

 Aaron Slack runs into Sty Head Tarn along the same line of fracture. 

 The col along this line, separating Great Gable from Green Gable, 

 is still only a few score feet beneath the lower summit. In course of 

 time it will be lowered until the two Gables form fells separated by a 

 deep gash, like Mickledore. 



Turning now to the consideration of mountains formed "in the 

 dry way," we may regard as typical examples the mountains of 

 Greenland, the peculiarities of which have been described and 

 explained by A. Kornerup. 2 Frost is the important agent at work 

 here, and the curve of stream-erosion is consequently absent. The 

 curved outlines of our lovely hills are replaced by monotonous 

 straight contours, (see plate vii. of Kornerup's Paper). These 

 contours are due to the frost working along the dominant joint-planes, 

 and consequently the outlines of the mountains depend on the 

 directions of the joints. It is true that among bedded rocks of 

 unequal hardness, dip-slopes and escarpments occur, but even here 

 the lines are straight and not curved, and when prominent planes of 

 stratification are absent, as among the gneissose rocks, the "house- 

 roof" type of mountain results from the action of denudation, 

 cirque-like valleys (Grydedal) do occur, but they are produced by 

 the action of frost on a system of curved joints. 



In Switzerland, many of the mountain faces show a similar 

 straight outline (it is well displayed on the cliffs of the Matterhorn), 

 indicating the efficacy of frost above the snow-line in determining 

 mountain contours ; but one may frequently notice what seems to be 

 the "ghost " of a stream-erosion curve, possibly pointing to the forma- 

 tion of some of the depressions between the higher mountains at a 

 period before the occupation of the area by snow and ice, though 

 subsequent frost-action has nearly destroyed them. 



Among the hills of desert regions, so far as one can gather 



2 " Meddelelser om Gronland," part ii., section vi. 



