FOSSIL GEOLOGY OF UNITED STATES. 295 
nothing remained but an oxide of iron, which was 
found to have taken the form of the deal so exact- 
ly, that even the dotted vessels so peculiar to this 
family of plants, and similar to those in the last cut, 
were distinctly visible under the microscope. 
Organic remains are usually coloured by the 
strata in which they are imbedded; in roestone, 
chalk, and the upper fresh- water limestone, they ap- 
proach a yellowish or brownish white ; in lias^ bitu- 
minous shale, and dark limestone, they incline to 
black ; and the shells in bituminous shale are some- 
times filled with bitumen in a fluid state. In the v 
uppermost strata, bones and shells retain their ori- 
ginal constituent parts very little changed, as in the 
fossil mammalia which we are about to describe. 
In the lower strata, the organic remains are more 
or less completely impregnated with mineral matter. 
The outer crust or shell of many chalk fossils is 
calcareous, and the internal part filled with flint. 
In some cases we find an external cast formed in 
the cavity of a crustaceous animal, and the exter- 
nal covering has disappeared ; in other instances 
the crust or shell of the animal has formed a mould 
in the stone, into which mineral matter has been 
subsequently infiltered, and thus made an external 
cast. Geologists have sometimes been deceived by 
mistaking the mineral nucleus formed on the inte- 
rior of the shell, for that which has received its 
impression from the>. outer shell. Thus, what are 
called fossil screws are the internal cast of a fossil 
univalve. 
It is impossible, within the limits of a work like 
the present, to give a complete account of the or- 
ganic remains hitherto discovered in our country. 
We shall therefore notice only such as will be most 
likely to interest the general reader. 
Fossil mammalia. — The extinct species of the 
higher orders of animals found fossil in the United 
States are those of the mammoth (rnastodon magoi" 
