FOSSIL GEOLOGY OF UNITED STATES. 296 



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 

 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 externaLcast 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 shaH. 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 (mastodon maxi- 



