GENERAL KEMAEKS OX GLACIATIOX. 99 



favorable. That the glacier, having been formed, carries away the detrital 

 material Avhich falls upon and delivers it at a distance from the place of its 

 origin, thus keeping the depression from becoming partly or entirel}^ filled 

 with debris, is a phenomenon which is of too simple a character to require 

 comment. However great the eroding power of the glacier may be after the 

 snow has become fully transformed into ice, there can be no question that it 

 must be very small as long as the material of which it is formed remains snow 

 or ntvt. Hence the utter inadequac}- of glaciers to carve out for themselves 

 the steeply walled cirques or amphitheatres in which they head, and which in 

 the Sierra Nevada often have almost exactlj'^ the form of a kettle, Avith the 

 rim broken away on one side. 



The occurrence of terraces as indications of climatic change will come np 

 for discussion in the next chapter, such forms of detrital accumulations being 

 the especial result of diminution in the amount of water passing down the 

 streams, or standing in the lakes, along whose banks they are found. Ter- 

 races are not unfrequently regarded as being in some way connected with 

 former glaciation. It is true that where, owing to change of climate, glaciers 

 have disappeared, there is likely also to have been a diminution of water in 

 all the adjacent region ; but moraines are the results of the work of the 

 advancing ice, while terraces are evidences of the retreating of the water. 

 These two forms of detrital accumulations have, however, a certain re- 

 semblance to each other, which has often led unpractised observers into 

 mistakes. 



Having thus in the preceding pages given a somewhat detailed description 

 of the character and extent of the manifestations of the former presence of 

 glaciers in the Cordilleras, the logical method of pursuing the subject would 

 seem to be to pass in review other portions of the earth's surface, in geograph- 

 ical order, and to furnish a similar statement for all those regions which exhibit 

 similar phenomena. It is evident, however, that such an undertaking would 

 require far more space than could be allowed in a volume like the present. 

 Besides, the facts are so well known and so easily accessible in a variety 

 of works, both general and special, that it is not necessary to go over the 

 ground again. All that needs to be done is to refer to and enlarge upon 

 such details of past and present glaciation, in certain regions, as may have a 

 special bearing on important points, and this will chiefly be done as these 

 points come up for discussion. A few very important districts will be de- 

 scribed with some fulness of detail ; while for most of the regions brought 



