OUTLINES OF GEOLOGY. 



85 



been protruded through fissures and faults, in the form of 

 dykes. The latter method alone is applicable to the por- 

 phyritic and ophitic groups, while the modes in which 

 the granite occurs are, in veins, in masses overlying or inter- 

 calated with the strata, and as the nuclei of mountains. 



192. The crystaline rocks of the inferior order, although 

 approaching to the granitoid in their structure and mineralo- 

 gical character, are stratified, and hence they are frequently 

 considered as having been deposited from water. It is not 

 necessary, however, to have recourse to this fluid in order to 

 account for their stratification, for we have seen that a 

 similar mode of arrangement is found in the craters of vol- 

 canoes, which also exhibit the phenomena of dykes, veins, 

 intercalated and overlying masses, so common in the gneissi- 

 form rocks. To these rocks, then, we would also ascribe an 

 igneous origin. 



In following the reliques of animal life, from the present 

 day to the lowest formations in which any trace of it exists, 

 the changes which have taken place in the inhabitants of our 

 globe present a curious spectacle. Each different order, 

 group, series, and even many single formations, have their 

 distinctive fossils. Only two genera seem to be common to 

 all, and these differ in species from formation to formation. 



The mammalia, even of the diluvium, differ in species, in 

 genus, and often in family, from those of the present day : 

 they are abundant in this and in the strata of the superior 

 order, but cannot be traced any farther, except in the single 

 instance of the family of marsupia. 



In the supermedial order, no trace of this class is found, 

 but reptiles are numerous, at first belonging to existing ge- 

 nera, but finally to families very different from those of the 

 present day. 



The remains of birds have been traced as low as this 

 order, but fishes are found as long as there is evidence of 



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