304 'iii 



llieiu are found peculiar species of shells, and alow corals, but a.s 

 yet, nowhere throughout the whole world, in this immense de- 

 posit called the Cambrian, (page 191), has the least fragment 

 been found referable to a fish; nor any traces of aquatic plants, 

 except a few sea- weeds; nor vcrtebrated animals. It is therefore 

 pretty evident, that at this time, either they did not exist, or if they 

 did, it was in extremely small numbers. We find the lowest or- 

 der of organization here developed, in the remains of a zoophyte, 

 or animal plant figured below, which is analagous to the sea-pen, 



and termed by Mr. Murchison, who has investigated with great 

 care, this series of rocks, graptolii&s, from the peculiar markings 

 of the stone; these were probably formed by an assemblage of a 

 vast number of individual polypes, each having a separate exis- 

 tence, but yet connected with the general mass, they are found 

 in great abundance in the.United States. There are also many 

 species of coral common in the most ancient rocks, and similar 

 in most respects to those now existing in the Indian Seas. The 

 low organization of these coral polypes has probably enabled them 

 to survive all the changes through which, so ofien the world has 

 passed, and we find them at the present moment as busily em- 

 ployed as in the earliest periods of the earth's existence. The 

 Silurian rocks are extensively developed over the whole world so 

 far as geologists have been enabled to explore. The immense de- 

 posits of calcareous flags, sandstones, shales, and limestones, 

 furnish our most valuable stones for economic purposes. As we 

 advance upwards in the series of deposits we find a marked 

 change not only in the mineralogical character, but in the num- 

 ber and form of the organic remains. Among the peculiar ani- 

 mals which flourished at this stage of the world's existence were 



