STUDIES FOR STUDENTS. 719 



extreme physical heterogeneity does not imply stratification, In 

 many cases drift composed almost wholly of earthy material, so 

 fine as to be popularly called clay, is wholly without stratification. 

 In such cases there are generally some stones associated, even 

 though they be small. Heterogeneity exists, but is not extreme. 

 In some places, though rarely, drift composed almost wholly of 

 sand, and therefore not remarkably heterogeneous physically, is 

 still without stratification. 



While the unstratified drift may be nearly homogeneous 

 physically, the stratified drift never reaches extremes of hetero- 

 geneity. It may be either coarse or fine, but the very coarse 

 and the very fine do not coexist. While there are occasional 

 beds of bowlders the relations of which suggest stratification, the 

 usual limit to the coarseness of stratified drift is coarse gravel. 

 Now and then large bowlders occur in the stratified gravel, sand, 

 or silt, but these bowlders cannot be regarded as partaking of the 

 stratified character which affects the matrix which encloses them. 

 They interrupt the stratification, as may often be distinctly seen. 

 They are accidents in the stratified deposits. No part of the 

 drift, as that term is generally understood, appears to be so fine 

 and so homogeneous as to fail of stratification because of its 

 fineness and homogeneity. The single apparently well-marked 

 exception to this statement (some parts of the loess) need not be 

 here considered. 



It is sometimes the case, as fresh cuttings through the drift 

 show, that lenses or pockets of stratified gravel or sand occur in 

 the midst of thick beds of unstratified drift. The reverse is also 

 sometimes true, considerable masses or chunks of unstratified 

 drift being now and then found in the midst of extensive strati- 

 fied deposits. This association, however, is much less common 

 than the other. Where the stratified drift occurs in extensive 

 beds, it may over.lie or underlie the unstratified, or the two may 

 alternate with each other repeatedly in vertical succession in a 

 single section. 



In some regions, the unstratified materials predominate greatly 

 over the stratified. In other regions the reverse is true. On the 



