THE PLEISTOCENE OR GLACIAL PERIOD. 353 



rock formations and of topography. From its physical make up we 

 know that the agency or agencies which produced it must have been 

 able to carry and deposit, at one place and at one time, materials as 

 fine as the finest silt or mud, and bowlders many tons in weight, while 

 they were competent, under other circumstances, to make deposits 

 of much less extreme diversity. From its lithological make up, and 

 from the nature of the finer parts of the drift, we know that the drift 

 forces worked on different sorts of rock, deriving materials from many; 

 that they ground some of the materials into a fine earthy powder or 

 " rock flour/ ' commonly called clay; that they as a rule derived the 

 larger part of the drift of any locality from formations near at hand; 

 and that the materials, even large bowlders, were sometimes carried 

 up to altitudes considerably above their source. From the structure 

 of the drift it is concluded that the drift force or forces must have been 

 capable of producing deposits which were sometimes stratified and 

 sometimes unstratified, and that the deposition of these two phases 

 of drift was sometimes contemporaneous and sometimes successive, 

 the number of alternations sometimes being considerable. From the 

 striae on the stones of the drift it is known that the production of the 

 drift must have involved the action of forces which, under some con- 

 ditions, were capable of planing and beveling and striating many stones, 

 especially the softer ones of the unstratified drift, while rounding and 

 leaving unstriated most of those of the stratified ; but that the agency 

 or agencies concerned must have been such that under certain cir- 

 cumstances their activities failed, on the one hand, to leave more than 

 a very small percentage of the stones of the unstratified drift beveled 

 and striated, while, on the other hand, they sometimes permitted 

 the stratification of gravels containing many subangular, plane-faced, 

 and striated stones, varying in size from pebbles to bowlders. From 

 the striae on the bed-rock beneath the drift and the unweathered char- 

 acter of the surface of the rock, it is clear that severe wear was inflicted 

 on the surfaces over which the drift was spread, while the positions 

 in which the striae were developed show that the agency which inflicted 

 the wear was able to adapt itself to all sorts of surfaces. The gen- 

 eral parallelism of striae in a limited area, and the systematic departure 

 from parallelism over great areas, are also significant of the manner 

 in which they were produced. From the topography of the drift it 

 is known that the forces which produced it must have been such as 



