OUTLINES OF GEOLOGY. 



21 



VI. 



OF ROCKS AND FOSSILS. 



38. The bodies which we find in the solid crust of the 

 earth are not wholly mineral. The remains of vegetable 

 and animal bodies are found imbedded in the upper beds, 

 often but little altered, and in other cases the forms of organ- 

 ized beings are distinctly visible, even where all trace of the 

 matter of which they were once composed has long since 

 disappeared. 



39. The mineral part of the earth's crust is made up of 

 masses, of greater or less extent, which, with the exception 

 of loose beds of recent origin, are called rocks. 



40. Rocks may be composed of a single mineral species, 

 or made up of several. Rocks are numerous, although less 

 so than the different mineral species, and their great variety 

 shows incontestibly the influence of many different causes 

 and modes of formation. 



41. Rocks which are of similar or probably nearly co- 

 temporaneous origin, are arranged in groups, which are 

 called formations. And these again may be considered as 

 greater or less, a great formation often including a number of 

 lesser ones. Great formations may again be arranged in 

 orders and series. 



42. Rocks rarely run into each other, and even contiguous 

 portions of the same rock may be divided from each other. 

 The lines of separation are called joints. 



43. Joints may be divided as follows, viz : 



(1.) Joints of texture, which depend principally on crysta- 

 line structure, and are therefore rather the objects of miner- 

 alogy than of geology; 



(2 ) Joints of stratification, which divide formations into a 

 number of layers, that when viewed throughout any consi- 

 derable extent are parallel to each other, although this paral- 

 lelism may not be perceptible within small limits. 



