STUDIES FOR STUDENTS. 7 l 7 



distribution of geological formations make it incredible that a 

 complex succession of identical formations may lie similarly in 

 a second direction from the given locality of drift, and yet be 

 wholly concealed. 



It is not to be understood that it is possible to trace every 

 bowlder of the drift to its exact source. Far from it. It is often 

 impossible to tell from what formation a bowlder came. An 

 unfossiliferous Cambrian sandstone may be indistinguishable 

 from an unfossiliferous sandstone from various other formations. 

 Even if it be possible to determine the formation from which 

 a given bowlder came, it does not follow that its exact or even 

 its approximate geographic source can be determined. It may 

 be possible, for example, to say that a given bowlder of the drift 

 came from the Laurentian formation. But the Laurentian forma- 

 tion is exposed over so great an area that it might not be possible 

 to tell, even approximately, the direction whence the bowlder in 

 question came, or the distance it has journeyed. In spite of these 

 limitations, there are some types of rock, and some associations 

 of types of rock, which are available for the determinations here 

 suggested. Drift bowlders derived from any formation possess- 

 ing distinctive characteristics, and exposed in a small area only, 

 give very definite information concerning both the direction and 

 the distance of their movement. Where complex associations of 

 bowlders occur, their joint testimony may be tolerably definite 

 on these points, even when that of each type, taken singly, fails 

 to be so. 



The fine material of the drift. The fine, earthy material of the 

 drift is popularly called clay. If it be critically examined, it 

 teaches significant lessons. It is found to differ in some essen- 

 tial respects from the mineral matter which makes up the soil 

 and subsoil of driftless territory. The latter is composed prin- 

 cipally of the insoluble ingredients of decomposed rock, while 

 the former is often nothing more nor less than pulverized rock, 

 the soluble constituents as well as the insoluble being present. 

 If it be examined under the microscope, tiny particles of all the 

 principal minerals which enter into the composition of any of the 



