xiv SURFACE GEOLOGY **7 



the stones (Fig. 84) ire often as well striated as the solid 

 rock below. Ix-t us pick out at random two or three 

 hundred stones from any section of boulder-clay or 

 moraine-stuff, and note down the proportions in which 

 each variety of rock occurs among them. We shall find 

 perhaps that 50 or 60 per cent may have been derived 

 from rocks in the immec&r it ao or 30 per 



cent have perhaps come a good many miles, while the 



remainder (usually small in size) may possibly be traced 

 to some of the most distant rocks in the drainage-basin 

 of the region. We learn from such an analysis the 

 tl direction of the ice-stream and see that it agrees 

 with the evidence furnished by the stria* on the rocks. 



Anfitnt va/ltys buried undfr Drift. Among the 

 interests of field-geology none appeal more forcibly to 

 the imagination than those which belong to the traces 

 of a vanished land-surface, side by side with the land- 

 surface of the present day. Reference has been nude 



