E. W. Hilgard—The Loess of the Mississippi Valley. 109 
ing the rootlets. Now it is not easy to see, how under such 
circumstances any shell consisting of calcic carbonate can 
remain undissolved. I here recall to mind my observations on 
the deposits of the later (Grand Gulf) Tertiary of the South- 
western States; where in a deposit evidently formed on the 
shores of the Gulf, consisting of fine-grained sandstones, clay- 
stones, and in some cases silts scarcely distinguishable from 
that of the loess period, we have such an absolute dearth of 
fossils that my most elaborate search in hundreds of localities, 
over an area nearly a hundred miles in width by two hundre 
sible clue to the distinction. The destructive processes are 
essentially dependent upon the presence and percolation of 
water; and this should be least in the marginal portion, where _ 
as a matter of fact most of the terrestrial fossils are found. 
Whether in addition, there is a difference as to destructibility 
* See my Memoir on the Geology of Lower Louisiana, and the Rock-salt De- 
posit of Petite Anse; Smithson. Contr., No. 248, 1872. 
