194 T. 8S. Hunt—The Decay of Rocks. 
abundant evidence, is less difficult to understand, when we 
Jearn that rocks as hard as those of our New York Highlands 
become [are] even in our own time, under certain conditions so 
softened as to offer little more resistance to the eroding action 
of a torrent than an ordinary gravel-bed." Subsequently, in an 
account of some observations made in North Carolina among 
the rocks of the Blue Ridge, and presented to the Boston 
removed by erosion during successive ages, culminating in the 
Glacial period at the close of the Pliocene, since which time the 
chemical decomposition of the surface has been insignificant. 
From the products of this sub-aérial decay it was then main- 
tained has been derived a great part of the sediments alike of 
Paleozoic, Mesozoic and Cenozoic times. The permeable nature 
rocks has been “a necessary preliminary to glacial and erosive 
action, which removed already softened materials.”"* Such erosion 
and denudation would, in accordance with this view, consist in 
18 The Nation, New York, Dec. 1, 1870. 
'  Pproe. Boston Soc. Nat. History, Oct. 15, 1873, and this Journal, vii, 60; 
also Proc. Amer. Assoc. Adv. Science for 1874, p. 39; and Hunt, Chem. and Geol. 
Essays, pp. 10, 250. . 
18 Harper’s Annual Record of Science, etc., for 1873, p. xlviii, 
+ 
