1877.] Surface Geology of Eastern Massachusetts. 583 
two are fashioned from stratified rocks, and are more regular and 
distinct than the others, which are for the most part composed 
of unstratified rocks. Yet the latter, no less than the former, 
reveal the structure of the rocks composing them; for exotic 
rocks, being, in a certain sense, structureless, only conform with 
the general law in giving rise to a systemless topography. 
The notion appears to be gaining ground among geologists 
that the power of a continental glacier to degrade the surfaces 
over which it moves, or, at least, to alter the forms of those 
surfaces, has been greatly over-estimated. It has become unnec- 
- essary, in the light of recerit investigations, to ascribe to the 
active agent of the drift epoch, whether land-ice or icebergs, 
great abrading power in order to account for the formation of 
the truly immense and generally chaotic mass of superficial de- 
tritus constituting the drift ; for in the subaerial decomposition 
of rocks, especially crystallines, in situ, during immense periods 
of time, we have a process fully competent for the production, 
both in quantity and quality, of the detrital materials, including 
bowlders, found in glaciated’ regions. The real degradation, the 
formation of the detritus, is mainly the work of chemical and 
not of mechanical forces. The sheet, usually thirty to forty, 
Sometimes fifty, and even one hundred feet in thickness, of 
thoroughly decomposed materials passing insensibly into solid 
tock below, found over a large portion of the Southern States, 
and oceurring generally wherever there are crystalline rocks in 
low latitudes, is a substantial monument to the degrading power 
of these silent agents, which are doubtless still in operation. As 
a nearly universal rule, we find the drift in New England repos- 
mg upon smooth and polished surfaces of undecomposed rocks, 
which evinces that the glaciating agent had sufficient erosive 
power to sweep away all traces of the zone of partially decom- 
posed, semi-rock-like material that in the South intervenes be- 
tween the firm rocks below and their decomposed skeleton above, 
| and which probably existed over glacial latitudes in preglacial 
times. The theory of subaerial decomposition so far diminishes 
the erosive power required, by previous hypotheses, in the agent 
of glaciation as to render possible a reconciliation of the exist- 
ence of an ice-cap in quite recent geologic time with the well- 
known fact that many reliefs of comparatively small magnitude 
have trends and contours wholly- at variance with the courses 
of glacial movement, and incompatible with the supposition that 
the ice-sheet moved as a rigid, unyielding rasp, removing hun- 
2 8 of feet of solid rock from the surface of the country. 
