340 



GEOLOGY. 



the deposits which it makes, and these are, on the other hand, pre- 

 cisely the features which existing glaciers are now impressing on their 

 deposits. 



(2) The bowlders and other stones of the drift. — The bowlders 

 and smaller stones of the unstratified drift possess significant 

 features. Many of them have smooth surfaces, but they are not 



Fig. 474. 



Pilot Rock." A bowlder of basalt near Coule City, Washington, 

 of the largest bowlders in America. (Garxey.) 



One 



generally rounded. They are often sub-angular, and the wear 

 which they have suffered has been effected by planing and bruising, 

 rather than by rolling (Fig. 254, Vol. I, and Figs. 476 and 477). The 

 plane sides meet one another at various angles, though the angle of 

 junction is rarely acute. These planed, sub-angular bowlders and 

 stones are often distinctly marked with one or more series of lines or 

 scratches, on one or more of their faces. The lines of each series are 

 parallel, but those of different series may cross at any angle. 



By no means all the stones of the drift show striae. They are rarely 



