T. 8. Hunt—The Decay of Rocks. . 209 
Potsdam sandstone in this vicinity, which is found to be 
converted to a depth of twenty feet or more into a plastic 
highly aluminous clay, which, from the presence of portions of 
titanium and chromium is, we may conjecture, derived from a 
doleritic rock.” 
Some cases converted into a claye e pyrites so 
abundant in the blue gravel, has, in these upper portions, or 
So-called red gravel, been oxidized, and the pict 
lignites have been silicified, and often incrusted with erystalli 
quartz, from silica liberated in the process of rock-decay 
through the infiltration of surface-waters.” 
, . To the porosity of the gravel, and the great amount of 
surface thus exposed, is to be added the influence of carbonic 
acid from the decaying lignite, the carbon of which is oxidized 
as the process of silicification goes on. The amount of carbonic 
dioxide in the air of certain drift-mines in these auriferous 
*4 Report Geol. Survey of Canada, 1878-79, H., p. 7. 
5 Geology of Canada, p. 896. Bikes 
56 LeConte, this Journal, xix, 177; Hunt, ibid, xix, 371. 
Am. Jour, Scr.—Taimp Serres, Vor. XXVI, No. 153.—SEPT., 1883. 
