200 STRUCTURAL GEOLOGY. 



into nodules of pure flint, and leave the chalk a pure 

 carbonate of lime deprived of its silica. Hence, chalk 

 usually contains flint nodules, scattered or in layers (Fig. 

 117). 



We speak of this nodular structure not on account of 

 its great importance, but because it is apt to strike the 

 observing eye, and very apt, too, to be mistaken for 

 fossils. 



Fossils : their Origin and Distribution. 



Every one must have observed that in many places the 

 stratified rocks contain the exact forms of organisms, 

 especially shells, though these seem to have turned to 

 stone. These are called fossils. They are of extreme 

 interest to geologists, because they reveal the nature of 

 the former inhabitants of the earth. Stratified rocks are 

 the consolidated sediments of former seas, bays, lakes, 

 and rivers. Then, as now, shells lived in the ooze of sea- 

 bottoms, or were cast up on beaches ; the leaves and 

 branches of trees and carcasses of land-animals were car- 

 ried down by rivers to lakes and estuaries and buried in 

 mud. These have been preserved, with more or less 

 change, to the present day. A fossil, then, may be 

 defined as any evidence of the former existence of a 

 living thing. Next to lamination, they are the most 

 constant characteristic of sedimentary rocks. 



Degrees and Kinds of Preservation. There are 

 various degrees and kinds of preservation of organic 

 forms. In some cases not only form and structure, but 

 even the organic matter of soft parts, is preserved. More 

 commonly, however, only the shells and skeletons of ani- 

 mals are preserved, and of these sometimes both iheform 

 and structure, and sometimes only the form. We shall 

 speak of these under three heads : 



1. Organic Matter preserved. This, of course, is 

 rare. The only perfect examples are those of carcasses 



