STUDIES FOR STUDENTS. 7 21 



of those whose attention has not been called to it. The true theory 

 of the drift must be one which will allow much of the unstratified 

 drift to have been subject to great vertical pressure, and perhaps 

 to shearing, at the time of its formation, or since. 



Shapes and Markings of the Stones in the Drift. 



If the stones in the drift be carefully examined, they are 

 found to possess significant features. If a goodly number of 

 them be collected from the unstratified drift, it will be seen that, 

 while their surfaces are often smooth, their forms are, on the 

 whole, somewhat unlike those of stones rounded by rivers or 

 by waves. While some of them are round or roundish, many 

 others are many-sided. Their faces are often beveled. They have 

 been worn, but the wear appears to have been effected by plan- 

 ing rather than by rolling. The planed sides may meet each other 

 at any angle, though the angle along the line of junction of two 

 faces is rarely sharp. With these planed and sub-angular bowl- 

 ders and stones there are associated few or many well-rounded 

 ones showing none of the characteristics just noted. With them 

 also, there are occasional angular masses of rock bounded by 

 fracture faces, which do not appear to have suffered notable wear. 



If an equally large number of stones from the stratified drift 

 be selected for examination, it will be found that rounded, water- 

 worn forms predominate. On the other hand, specimens of the 

 many-sided, plane-faced forms, though much less common than 

 in the unstratified drift, are not always altogether wanting. This 

 distinction affects large and small stones alike. It characterizes 

 bowlders a foot in diameter, if so large stones occur in the strati- 

 fied drift, and it characterizes fragments less than an inch in 

 diameter. In case laro;e bowlders occur in the stratified drift, 

 they are rather more likely to have the sub-angular form than the 

 smaller ones. 



Another peculiarity goes along with the foregoing. The 

 planed, sub-angular bowlders and rock fragments which char- 

 acterize especially the unstratified drift, are often distinctly marked 

 with one or more series of lines or scratches on one or more of their 



