No. 4.] CLAYPOLE — PRE-GLACIAL GEOGRAPHY. 215 



down into wide, shallow valleys; another, of different structure, 

 shows a rugged outline of cliff and gorge, of plateau and canon. 

 In the former case erosion is so rapid that the streams are unable 

 to carry away the material as fast as it is broken down by the 

 weather. The erosive agents surpass the transporting. In the 

 second the rocks afford less material than the streams can re- 

 move, and the transporting power exceeds the eroding. Hence 

 come the two great types of surface-contour, the rugged and 

 the smooth, the former characteristic of resisting, the latter of 

 yielding material. It must therefore evidently follow that a 

 river flowing through regions composed of rocks of both kinds 

 will produce alternately the broad open vale and the deep narrow 

 chasm. 



But apart from theory, Elisee Reclus (The Earth, p. 132) says : 

 *' Some valleys present a succession of rounded basins separated 

 from each other by narrow passes. In the Pyrenees, the Jura 

 and the calcareous regions of the Alps, valleys of this kind are 

 very numerous." " The variations in the shape of valleys may 

 be explained by the different natures of the rocks which the 

 waters have had to hollow out. Wherever the materials operated 

 upon — gravel, sandstone, granites, schists or lavas — are of ana- 

 logous composition, and thus everywhere present an equal resist- 

 ance to the action of the water, the latter is able to pursue its 

 normal movement and adopts a meandering course. On the 

 contrary, where the rocks consist of strata of unequal hardness, 

 or are traversed by obstructing walls, the water is necessarily 

 compelled to spread out into a lake-like accumulation, in the 

 meantime eating away the banks in a lateral direction, until the 

 barrier being at length penetrated, the sheet of water is poured 

 down to some lower level. In this way there has been formed 

 during a course of ages a series of basins one above another, 

 some of which are still partially filled with water, others entirely 

 empty, all being linked together by narrow defiles through which 

 pours the mountain torrent." 



In years past the writer resided on the banks of the Avon in 

 England. This river affords a remarkable illustration of the 

 same phenomenon. In its upper course it flows over the Oxford 

 clay in a wide open vale or plain, but before reaching the city of 

 Bath it comes upon the harder beds of the great oolite through 

 which its course lies in a deep valley bordered by high hills of 

 that formation, to which the city owes much of its attractiveness 



