710 THE JOURNAL OF GEOLOGY. 



rials of which it is composed did not originate where they now 

 lie, and that, in consequence, they sustain no definite genetic rela- 

 tionship to the formations of the territory over which they are 

 spread. Long before the drift received special attention at the 

 hands of geologists, it was believed that it had been transported 

 from other localities to those where it now occurs. The early con- 

 ception was that it had been brought or " drifted " to its present posi- 

 tion from some outside source by means of water. It is to this con- 

 ception of its origin that the formation owes the name drift. In 

 the early days of geology this surface material, which often effect- 

 ually conceals the rock beneath, was regarded as uninteresting in 

 itself, and an obstacle to the study of the underlying formations, 

 which were regarded as the proper field of geological inquiry. 

 So long as the drift was looked upon in this way, it received but 

 little attention ; but within recent years it has been the object of 

 critical investigation, and there are now few departments of geology 

 which are attracting a larger share of professional attention, and 

 few departments which have yielded, or are yielding, more inter- 

 esting and more important results. The accessibility of the drift, 

 and its importance in shaping the details of the surface through- 

 out so wide an area, have made it a favorite subject of study for 

 a large number of students. 



Thickness of the Drift. 



In ge?ieral. The thickness of the drift varies greatly. Over 

 large areas its depth is so slight and so unequal that the under- 

 lying rock is frequently exposed. This is much more generally true 

 in mountainous and hilly regions than in plane ones, though 

 regions are not wanting where frequent rock exposures are associa- 

 ted with a topography of but slight relief. The mountains of New 

 England, the Adirondacks, the Catskills, the Highlands of North- 

 ern New Jersey, and the adjacent parts of New York and Penn- 

 sylvania, may serve as examples of mountainous regions where 

 the underlying rock is but poorly concealed by the mantle of 

 drift. Areas of plane or gently undulating topography where 

 the drift is so thin as to allow the underlying rock to be exposed 



