POPELAE EXPLANATION. 15 



or other of finding exposures of each one at the surface. In 

 Iowa the order of superposition of the different formations is 

 comparatively simple because no great disturbance of the 

 strata has taken place here to cause confusion in their identi- 

 fication, as has been the case in some other, particularly in 

 mountainous regions. 



Nearly all stratified rocks, as before intimated, are found 

 to contain the fossil remains of various kinds of animal and 

 vegetable forms, These in our rocks are principally shells 

 and corals of many kinds, and in some of them plants also, 

 all of which existed while the sediment was accumulating, of 

 which the rock that imbeds them was formed. 



By the study of the fossil remains of all these formations 

 the geologist finds that certain formations or assemblages of 

 strata are characterized by certain fossils peculiar to them 

 alone, and by which he is able to recognize the same wherever 

 they may be found, even if the character of the rock itself is 

 wholly different at different points. The order of one above 

 the other and relative position of formations thus character- 

 ized is never reversed; never changed. Consequently, if but 

 a single formation is found exposed anywhere, its proper 

 position in the great scale of formations is as well known as 

 if all the others were exposed to view in contact with it, and 

 in their regular order. It is this that gives collections of 

 fossils their great value, for without the information they 

 thus afford, they would possess no more real value than the 

 broken shells upon the shores of existing seas ; and without 

 that information, which we derive from that source alone, 

 geology as a science could not exist. 



The strata thus studied are, in consequence of certain char- 

 acteristics possessed in common by a greater or less number 

 of contiguous ones, divided into formations. A greater or 

 less number of these contiguous formations constitute groups, 

 and a greater or less number of these groups constitute sys- 

 tems, which are named in the order of their relative age. 

 These systems, beginning with the oldest and geologically 

 the lowest, are the Azoic, Lower Silurian, Upper Silurian, 



