STUDIES FOR STUDENTS. J I 5 



Litho logical heterogeneity . The drift of any given area gener- 

 ally contains pieces of some such assemblage of rocks as the fol- 

 lowing : shale, sandstone, limestone, quartzite, gneiss, schist, 

 porphyry, diabase, and gabbro. Not all these types of rock are 

 represented in the drift of every locality, and some localities 

 contain rock fragments of other types. But if all the foregoing 

 kinds of rock are represented in the drift of a given locality, the 

 drift containing them cannot be looked upon as possessing unusual 

 lithological heterogeneity. Any type of rock represented in the 

 drift may be represented by masses as large as the largest bowl- 

 ders which the drift contains, and, at the same time, by the smallest 

 particles which will allow of identification. In other words, the 

 physical heterogeneity of any given sort of rock represented in 

 the drift may be about as great as the physical heterogeneity 

 of all. 



The various types of rock represented in the drift of any 

 locality are not generally in equal proportions. It is often true 

 that some one type predominates, and this predominance is 

 often striking. But in spite of this, the lithological hetero- 

 geneity of the drift is a well-nigh universal characteristic. Any 

 hypothesis which essays to explain its origin must take account of 

 this fact. 



We must suppose that the granite of the drift came from 

 some place or places where granite is the native surface rock 

 since we know of no other possible source for it. We suppose 

 that the shale, sandstone, and limestone of the drift came from 

 regions where shale, sandstone, and limestone severally occur, as 

 the uppermost formations beneath the drift. But so many sorts of 

 rock as frequently occur in the form of loose bowlders in the drift 

 of a given place, do not often occur as surface formations in 

 closely contiguous localities. Within the area covered by the 

 drift there are rarely more than a limited number of rock forma- 

 tions appearing at the surface in closely associated areas. 

 From its lithological heterogeneit}', therefore, we must conclude 

 that the sources of the drift were various and wide-spread ; and 

 if we conclude that its sources were wide-spread, we have no 



