Fossils. 153 



The primitive earths form stones, as 

 we have mentioned, and stones united by 

 cement form those masses called rocks. 

 We shall notice the six mixtures which 

 are most commonly found in those mas- 

 ses, viz. porphyry, serpentine, ophites 

 granitell, granite, andjtint. 



Porphyry is composed of felspar in 

 small fragments, of schorl, and a kind ot 

 cement, which unites ail the parts, and 

 which, in some measure, forms the base. 

 Porphyry is exceedingly hard, and diffi- 

 cult to be cut j it will, however, take a fine 

 polish. Some kinds of it are red, and 

 others green. 



Serpentine is composed of the same 

 substance as porphyry. The only differ- 

 ence is, that the felspar is in larger frag- 

 ments. The colour of serpentine is vari- 

 ous ; some kinds are green, others violet, 

 some yellow, and some black. 



The ophite is composed of only two 

 substances, viz. black schorl, known un- 

 der the name of ancient black basaltes, 

 interspersed with greenish felspar, which 

 forms in it long spots. This stone has 

 considerable hardness. 

 N2 



