18 STUDIES, SCIENTIFIC AND SOCIAL chap. 



and hence the reason why valleys are in some parts very 

 narrow and precipitous, in others wide and open. It is 

 an invariable rule that hills and mountains are composed 

 of the harder or less soluble rocks, the adjacent lowlands 

 and valleys of the softer and more soluble. Hence, we 

 see all great mountain ranges mainly composed of the 

 older, hard, or crystalline rocks, while the lowlands, plains, 

 and valleys are occupied by the newer and softer forma- 

 tions. In our own country the tertiary or secondary 

 clays and sands are found in the lowland districts, while 

 the more ancient and much harder rocks form the hills of 

 Devonshire, Wales, the Lake District, and Scotland. 



Keeping in mind the extreme inequality of the rate of 

 denudation of different rocks, we are able easily to 

 explain the apparently erratic course of so many rivers. 

 When the streams originated they took their course along 

 lines of least resistance, depending on the form of the 

 surface, not on the nature of the rocks beneath the 

 surface. Sometimes this course passed over ridges or 

 bosses of very hard rock, buried perhaps hundreds, perhaps 

 thousands, of feet deep. But the channels once fixed 

 could not be altered, and when the bed of the stream 

 reached this rock it cut down into it. Then, owing to the 

 hardness of the rock, the river channel would be a gorge 

 or ravine, while all around the softer rocks would be 

 denuded by frost and rain, so that extensive areas would 

 be lowered as fast as the stream cut its narrow channel 

 through the hard rock, and was able to carry away the 

 denuded material ; while the gravel or sand thus carried 

 down assisted in wearing away and cutting out the ravine 

 through which it all had to pass. Hence, in the course of 

 ages we should have the stream flowing over a wide low- 

 land, perhaps on one side open to the sea, and then 

 cutting straight across a mountain ridge, or even across 

 an isolated hill entirely surrounded by lowlands. 



Not very much time, geologically speaking, is required 

 for such operations. Sir Charles Lyell describes a channel, 

 cut by the river Simeto across a lava stream from Etna, 

 which is over fifty feet wide and in some parts forty to 

 fifty feet deep. The lava is not porous, but is a homo- 



