STRATIFIED ROCKS. 



199 



marked with polygonal cracks, simulate very much a 

 turtle-shell, and are called turtle-stones ; sometimes dumb- 

 bell-shaped, sometimes rings, sometimes all sorts of 

 strange and fantastic shapes (Fig. 116). In structure 

 they are sometimes solid, sometimes hollow, sometimes 

 affected with interior cracks, sometimes have a concentric 

 shell-structure, and sometimes a radiated structure. 



These curious shapes so simulate fossils that even ex- 

 perienced geologists may sometimes be in doubt. By 

 common observers they are very often mistaken for fossil 

 nuts, fossil turtles, etc. They are, however, very inter- 

 esting to the geologist, because they often contain a fossil 

 beautifully preserved in the center. 



How Formed. They seem to be formed by the slow 

 aggregation of more soluble or more suspensible matter 

 from a general mass of insoluble matter, an organism 



FIG. 117. Chalk-cliffs with flint nodules. 



often forming the nucleus of aggregation. Thus, if the 

 mass be a calcareous sandstone, the lime will gather in 

 places, forming sandstones containing more lime than the 

 general mass. So calcareous clays form nodules of lime 

 mixed with clay. These are the hydraulic-cement nod- 

 ules. In chalk the disseminated silica seems to gather 



