714 THE JOURNAL OF GEOLOGY. 



areas are notably higher or lower than their surroundings, though 

 both are notably rougher. 



In considerino"the origan of the drift, we are forced to exclude 

 as the sole or principal agents such as would not allow the exist- 

 ence of driftless areas, neither notably higher nor lower than their 

 surroundings, within the very heart of the great sheet of drift. 



Constitution of the Drift. 



Physical heterogeneity. It is certainly a striking fact which 

 confronts us when we see huge bowlders, sometimes many tons 

 in weight, imbedded in earthy material as fine as that which our 

 most sluggish streams are carrying in suspension. Between these 

 extremes of coarseness and fineness, there are materials of all 

 grades. The proportions of coarse and fine materials are not at 

 all constant. The fine may predominate at one point, the coarse 

 at another. From predominance of fine to predominance of 

 coarse the changes may be abrupt, and frequently repeated. Any 

 one of the constituents of the drift — bowlders, gravel, sand or 

 clay — may predominate over any or all the others to almost any 

 extent. It follows that any one of these may nearly or quite 

 exclude the others, though the drift is rarely composed of large 

 stones only. While, therefore, the drift is remarkable for its 

 physical heterogeneity, and while this heterogeneity is a general 

 characteristic, still there are localities where it is not extreme. 

 There are localities, indeed, where the drift is remarkably homo- 

 geneous. In such cases its constituents are more commonly fine 

 than coarse, and rather more commonly of sand than of clay, 

 although drift made up principally of the fine, earthy material 

 which is popularly called "clay," is by no means rare. 



From the physical heterogeneity of the drift, it is clear that 

 the agent or agents to which it owes its origin, or to which at any 

 rate much of it owes its origin, must have been able, under some 

 conditions, to carry and deposit at one place and at one time, 

 materials as fine as the finest particles of silt or mud, and bowl- 

 ders many tons in weight, while they were competent, under other 

 circumstances, to make deposits of much less extreme diversity. 



