852 



GEOLOGY 



one or more series of lines or strice on one or more of their f;i 

 The lines of each series are parallel, but those of different sets may 

 cross at any angle. By no means all the stones of the drift show 

 striae. They are rarely seen on those which have lain long at the 

 surface, and they are more common on the less resistant sorts of 

 rock, such as limestone, than on more resistant ones, such as 



Fig. 564. Stones of the drift, striated and beveled by glacial wear. (U. S. 



Geol. Surv.) 



quartzite. No depositing agent except glaciers habitually marks 

 the stones which it deposits in this way. 



3. Structure. The larger part of the drift is unstratified, but 

 a considerable part is stratified, often irregularly. The unstratified 

 drift (Fig. 565) or till (for some of it the name bowlder-clay is appro- 

 priate), has little orderly arrangement of its parts, yet it often lias 

 a sort of rude cleavage which has been called foliation (Fig. 566). 

 The planes of cleavage are in such position as to suggest that they 

 were developed by pressure from above. The stratified drift 

 (Fig. 567) shows by its structure that it was deposited by water, 

 which doubtless sprang, in large part, from the melting of the ice. 



