7 1 8 THE JO URNAL OF GEOLOGY. 



bowlders of the drift of the same locality may be recognized 

 In the clayey part of the drift, the dolomite and calcite of the 

 limestone, the sand grains of the sandstone, the quartz, orthoclase, 

 plagioclase, mica, hornblende, augite, olivine, magnetite, etc., and 

 even the rarer apatite, tourmaline, zircon, garnet, etc., of the crys- 

 talline rocks, may generally be found in microscopic particles, 

 if bowlders containing these minerals are at all plentiful. In 

 general, the minerals which are abundantly represented in the 

 drift bowlders of any locality, are relatively abundant in the fine 

 earth of the drift of the same locality, while the rarer minerals 

 of the bowlders are correspondingly rare in the accompanying 

 clay. To this general statement there are some exceptions. 

 The mineral particles of which the fine parts of the drift are 

 composed often appear as fresh as if but just broken from the 

 crystals of undecomposed rock. Where the particles are large 

 enough so that their forms and surfaces can be distinctly made 

 out, they are seen to be predominantly angular in shape, and 

 to be bounded by fracture faces. They are just such particles as 

 might result from grinding up together various sorts of rock. 

 Furthermore, the fine earthy matter of the drift of any locality 

 is of such mineralogical composition as to suggest that it was 

 largely made from the grinding up of just those sorts of rock- 

 which are now represented in the large and small stones of the 

 drift. The physical nature of the earthy matter of the drift, 

 taken together with its mineraloaacal and chemical constitution, 

 leaves no room for doubt that it is nothing more nor less than 

 "rock flour," the product of mechanical grinding. The agencies 

 which produced the drift must have been able to pulverize rock. 

 No theory which does not take this into account can be acceptable. 



Structure of the Drift. 



Stratification. Much of the drift is distinctly stratified, and 

 much of it is altogether devoid of stratification. Its structure is 

 altogether independent of lithological heterogeneity, but is inti- 

 mately connected with physical heterogeneity. Extreme physical 

 heterogeneity implies absence of stratification, but absence of 



