1882.] 25 [Davis. 



Riitimeyer calls the period of glacial occupation, the pupa-stage of a 

 valley's history, for the above reason. (Ueber Thal-und Seebildung, Basel, 

 1869, 24.) Kinahan says that glacial erosion is effective only where pre- 

 glacial faulting, jointing, and weathering have loosened the bed-rock so 

 that the ice has only to scrape away the prepared fragments : he notes the 

 reduced rate of erosion after this has been accomplished and solid rock 

 reached. (Valleys and their relation to Fissures, Fractures, and Faults ; 

 London, 1875, 120, 124.) 



J. D. Whitney considers glaciers the carriers but not the originators of 

 morainic material. (Climatic Changes, 1881, 7.) 



A. 5. Limit of Time. All the preceding paragraphs show 

 that a certain amount of erosion goes on beneath the ice, and 

 this is confirmed by the milky appearance of the subglacial stream 

 where it runs out at the end of the glacier, charged with rock 

 flour; and quite unlike the limpid streams on the neighboring 

 valley-sides, flowing from melted snow or from springs. But the 

 comparison often made between the work of these two classes of 

 streams, to the disadvantage of the latter, is hardly just. Brooks 

 from springs are busy enough carrying down detritus just after a 

 rain, although they have a rest from heavy work in fine weather : 

 their action is intermittent, instead of almost continuous like that 

 of the subglacial streams, but there has not yet been shown to be 

 any great difference in the total results of the two ; and certainly 

 nonglacial streams have been active enough in all parts of the 

 world, as is known by the results of their cutting. 



But, in the face of the continual grinding that goes on beneath 

 glaciers, to argue that they could never erode valleys, savors of 

 the same conservatism that stood so long in the way of a proper 

 understanding of the work of streams and rivers. These have 

 done little in our short historic period, but have been admittedly 

 very effective during their long persevering life. And almost 

 equally effective would glaciers have been had they worked as 

 long. We cannot deny their power, but we may say that the 

 time through which they have acted has been insufficient to 

 produce great results. (See below, C. 4). 



A. 6. Subglacial Streams. A certain amount of rock erosion 

 must be admitted and we may now inquire where this took place 

 most rapidly. All that is due to subglacial streams will be closely 

 limited to the lines of drainage of the subolacial surface, and 



