PRIMARY AND TRANSITION ROCKS. 



7 



without any material change of the present names, and he is persuad- 

 ed that such an arrangement will take place in a more advanced stale 

 of the science, (see Chap. V.) 



Primary or Primitive Rocks — were so called because no fossil re- 

 mains of animals or vegetables, nor any fragments of other rocks, 

 were found imbedded in them : hence it was supposed that they 

 were formed prior to the creation of organic beings. The rocks of 

 this class are for the most part extremely hard, and the minerals of 

 which they are composed are frequently more or less perfectly crys- 

 tallized. These rocks generally occur in immense masses or beds; 

 they form the lowest part of the earth's surface with which we are 

 acquainted, and they not only constitute the foundation on which 

 rocks of the other classes are laid, but in many situations they pierce 

 through the incumbent rocks and strata, and form also the highest 

 mountains in alpine districts. We are not to conclude, when we 

 see a mountain or range of mountains bounded by a plain, that the 

 mineral beds and strata of which these mountains are formed term- 

 inate at their apparent bases ; on the contrary, they dip under the 

 surface at angles more or less inclined, stretching below the lower 

 grounds and hills, and often rising again in remote districts. 



That primary rocks environ the whole globe will not admit of di- 

 rect proof ; but, from their frequent occurrence in mountainous dis- 

 tricts in the most distant parts of the world that have been examined, 

 we may infer that some of the rocks of this class constitute the foun- 

 dation rock of every country. We have no means of ascertaining that 

 the similar rocks of distant districts were formed at the same time, 

 nor can we be certain that the rocks called Primary, have not once 

 contained organic remains, that were destroyed during the process 

 by which they acquired their present crystalline structure. We may 

 however, with apparent probability, infer that their formation was 

 prior to the existence of animals or vegetables on our planet in its 

 present state, because the rocks which immediately cover them con- 

 tain almost exclusively the organic remains of the lowest class of an- 

 imals, which are considered as forming the first link in the chain of 

 animated beings. On this account these rocks have been called by 

 the German geologists transition rocks, from the supposition that they 

 were formed when the world was passing from an uninhabitable to a 

 habitable state. 



Transition or intermediate rocks are generally less crystalline than 

 the primary ; they contain occasionally organic remains of the lower 

 classes of animals, and also fragments of rocks of the primary class. 

 They are frequently interposed between rocks of the primary class, 

 and those more generally called secondary, and often partake of the 

 character belonging to both. The prevailing rocks in the transition 

 series are limestone, slate, called clayslate, and coarse slate, passing 

 sometimes into sandstone, and conglomerate ; this has been called 

 by the Germans grau wacke, or grey wacke. The rocks of the pri- 



