Dr. T. Sterry Hunt—On the Decay of Rocks. dll 
spheric agencies. Having first discussed the chemistry of the process, 
he noticed the production of spheroidal masses, or so-called boulders 
of decomposition, by the decay and exfoliation of massive rocks. 
He then proceeded to show that the process of decay is not, as some 
have supposed, a rapid or a local one, dependent on modern conditions 
of climate, but that, on the contrary, it is universal, and of great 
antiquity, going back into very early geological periods. These 
conclusions were supported by details of many observations among 
Paleozoic stratified and eruptive rocks in the St. Lawrence valley, 
as well as among Hozoic rocks in the Atlantic belt, as seen in Hoosac 
Mountain, in the South Mountain, and in the Blue Ridge. In con- 
nection with the latter he described the decay, not only of the 
crystalline strata, but of their enclosed masses of pyritous ores, and 
the attendant phenomena. The decay of the primal and auroral 
strata of the Appalachian valley, and the formation therein of clays 
and of iron and manganese oxides, was also discussed. The pre- 
Cambrian antiquity of the process of decay in the Hozoic rocks of 
the Mississippi valley, as shown by Pumpelly and by Irving, as 
well as similar evidence from Europe, was noted, while the more 
recent decomposition seen in the auriferous gravels of California 
was described and explained. 
The final removal of the covering of decayed rock from many 
northern regions during the drift period was then considered ; and 
the thesis advanced by the speaker in 1873, that the decay of rocks 
‘is an indispensable preliminary to glacial and erosive action, which 
removed previously softened materials,” was discussed in its rela- 
tions to boulders, glacial drift, and the contour of glaciated regions. 
Pumpelly’s development and extension of this doctrine to wind- 
erosion was noticed, and also the recent comparative studies of 
Reusch in Norway and in Corsica, in which similar views are en- 
forced. 
The principal points in the paper, as viewed at its close, are as 
follows :— 
1. The evidence afforded by recent geological studies in America, 
and elsewhere, of the universality and the antiquity of the subaerial 
decay, both of crystalline silicated rocks and of calcareous rocks, and 
of its great extent in pre-Cambrian times. 
2. The fact that the materials resulting from such decay are pre- 
served in situ, in regions where they have been protected from denu- 
dation by overlying strata, alike of Cambrian and of more recent 
periods ; or, in the absence of these, by the position of the decayed 
rock with reference to denuding agents, as in driftless regions, or in 
places sheltered from erosion, as within the St. Lawrence and Appa- 
lachian valleys. 
3. That this process of decay, though continuous through later 
geological ages, has, under ordinary conditions, been insignificant in 
amount since the Glacial period, for the reason that the time which 
has since elapsed is small when compared with previous periods ; 
and also, probably, on account of changed atmospheric conditions in 
the later time. 
