STUDIES FOR STUDENTS. 7°9 



to any extent, or all maybe commingled in approximately equal 

 proportions. 



Through this mantle of unconsolidated material the underlying 

 rock often projects. Many natural and artificial sections like- 

 wise reveal the rock beneath. From these sections, and from 

 the general relations of the drift to the underlying rock, it is seen 

 that the surface materials are not restricted to any particular sort 

 of rock. They occur on limestone, sandstone, shale, gneiss, 

 granite, or any other sort of rock, with apparent indifference. 



Another feature which at once attracts attention is the fact 

 that the body of material overlying any particular kind of rock 

 contains many fragments or bowlders of rock which could not have 

 been derived from it. Where the underlying rock is limestone, 

 bowlders of gneiss, granite, sandstone, or diabase are often found 

 in abundance in the overlying mantle of loose materials. Such 

 bowlders cannot be supposed to have come from the disruption 

 of the limestone, for limestone does not contain the materials of 

 which they are composed. In like manner, the covering of uncon- 

 solidated material which overspreads the surface of gneiss 

 within the area specified, often contains bowlders from a great 

 variety of other formations, such as limestone, sandstone, and 

 shale. Disintegration or disruption of the gneiss could by no 

 possibility have given rise to limestone or sandstone or shale, 

 since gneiss contains nothing from which these rocks could come 

 by any simple process of disintegration or disruption. Not only 

 does the composition of the loose materials overlying the solid 

 rock in the northeastern part of the United States forbid the idea 

 of their origin by the decay of the underlying rock, but in 

 addition to this, the physical condition of the bowlders has a like 

 significance, since many of them show no signs whatever of dis- 

 integration. They often look as fresh as if quarried but yester- 

 day from their parent formations. 



This aofareorate of surface material which overlies different 

 formations indiscriminately, and which is composed of materials 

 which could not have been derived wholly from the underlying 

 rock, is called drift. It was long since recognized that the mate- 



