210 T. S. Hunt—The Decay of Rocks. 
gravels is so great that candles will not burn therein. Mr. D. 
ughes of San Francisco, a well-known mining engineer, to 
whose careful scientific observations I have been 
debted, informs me that in the case of a drift-mine 300 feet 
below the surface, in Table Mountain, Tuolumne Co., Cal. 
where the foulness of the air was especially painiinied: he satis- 
_ fied himself by appropriate tests of the presence in the air of a 
large proportion of carbonic-dioxide. If, as there is reason to sup- 
pose, the amount of this element in our atmosphere was somewhat 
greater in former ages than at present, we have in these gravels 
y by Professor Pumpelly on Secular Rock- 
Ga tncteration, read before the National Academy of Sciences — 
in April, 1878,” is a very valuable contribution to the subject 
before us. He cites therein m oo conclusions as to the great 
t 
seeual ave te 
