214 PREVAILING THEORY CONCERNING [Ch. XI. 



The first division comprises the different kinds of granitic 

 rocks ; the second, the traps and porphyries ; the third, the 

 various ejecta of volcanos. 



On this view of the subject, it is evident that all kinds of 

 igneous rocks may have been produced at different periods, 

 and are still actually forming. This theory is supported by 

 the fact, that trap and volcanic rocks are associated with de- 

 posits which decidedly belong to distinct epochs : but the 

 evidence in the case of granite is not so decisive ; and it there- 

 fore requires to be stated more in detail. The strata with 

 which granite is intimately connected, in the form of veins 

 and dykes, are generally more or less crystalline; and in 

 some cases, so much so that they cannot be mineralogically 

 distinguished from the granitic rocks; they are free from 

 organic remains ; in short, what are commonly called primary 

 rocks. In some instances, these rocks are more ancient than 

 the oldest fossiliferous strata, because the latter are composed 

 of the detritus of the former : it is, however, contended by 

 some, that this is no proof that the existence of animated 

 beings commenced with the present fossiliferous strata, since 

 these primary slates themselves may have been originally 

 sedimentary rocks which once contained the remains of ani- 

 mals still more ancient than those which have been hitherto 

 discovered. The obliteration of all traces of these creatures, 

 according to the theory, has been occasioned by these strata 

 having been in contact with granite, in a state of igneous 

 fusion, for a period of considerable duration: this elevated 

 temperature, however, was not sufficient to melt the strata, 

 for they still retain their original structure; but to impart 

 such a freedom of motion to their constituent particles, as to 

 enable them to assume a crystalline structure on the refriger- 

 ation of the mass. 



Now, according to these views, primary strata, not to be 

 distinguished from those which are older than the secondary 

 rocks, could be produced from any group of the secondary or 

 tertiary formation, with which the protruded igneous granite 



