268 Scientific Intelligence. 
thickness of rather more than two hundred feet. The whole deposit is 
of comparatively modern date, the upper sixty feet consisting of clayey 
m, containing shells of recent species, and the lower of sand and 
gravel without organic remains, except some wood and silicified corals 
washed out of older rocks. ‘The yellow loam at the top bears a singu- 
larly close resemblance to the fluviatile silt, or “ loess,” as it is termed, 
of the valley of the Rhine, between Cologne and Basle, and, like it, 
contains abundance of freshwater and land shells, of which I myself 
obtained more than twenty species, now in my cabinet in London. 
States. With these shells are found, at different depths, some in the 
loam, and some in the clay at, the bottom of the loam, the scattered 
bones and occasionally entire skeletons of the mastodon, megatherium, 
mylodon, casteroides, equus, bos, and other quadrupeds. If in any 
had lived in North America when the geographical configuration of 
the country was very different,—in other words before the valley of 
the Mississippi, or even sorne of the strata forming its boundary rocks, 
were in existence. Every deposit entering into the composition of the 
bluffs of the Mississippi valley must be older than the alluvial plain and 
delia of that river ; and we thus obtain, independently of any evidence 
or we must allow a sufficient series of years for the depositioa of all 
to 
and the formation of its alluvial deposits and delta? The yellow 
shelly loam or loess, before mentioned, extends for twelve miles inland 
or eastward from the river; and in consequence of its i rent and 
destructible nature, every streamlet flowing over it euts out for itself, 
in its way to the Mississippi, a deep gully or ravine. This denudation 
has of late years proceeded with accelerated speed, especially in the 
”—ooOoO ee ae ae eres 
* 'This volume, p. 34. 
