194 GENERAL REMARKS [Ch. X. 



rocks, but also the relation of these towards each other : thus, 

 although the so called primary rocks constitute a distinct 

 group, under whatever circumstances they occur, yet that they 

 are not always referrible to the same geological epoch, inas- 

 much as they are not only connected with and graduate into 

 the older groups of the secondary class, but are also similarly 

 associated with those of the most recent of the same class, or 

 even with the tertiary deposits. 



From this statement it follows that the granitic groups are 

 of different ages, a conclusion which in a great measure seems 

 to depend on the nature of the passage of these rocks into 

 those of the fossiliferous strata. The value of such a passage, 

 as a criterion of the identity of rocks between which it occurs, 

 will form one of the topics in the succeeding discussion : in- 

 deed, it is now time to enter on an examination of the pre- 

 vailing theory as applicable to the primary rocks ; for the next 

 step in the investigation, after having obtained a description 

 of our subject, is to ascertain the true position and age of 

 these rocks in reference to the other masses of which our 

 planet is composed. But before entering on this considera- 

 tion it will be desirable, by way of recapitulation, to review 

 the leading points of the preceding descriptions, so as to ob- 

 tain one general view of the primary rocks, divested, as much 

 as possible, from all hypothesis. 



Wherever the primary formations are extensively exposed 

 to view, they are found to consist of granitic and schistose 

 rocks : the former, for the most part, are composed of two or 

 more crystalline minerals, which are generally aggregated in 

 such distinct concretions that their nature can be easily ascer- 

 tained, though, sometimes, the mixture is so fine that its 

 constituents can only be detected by tracing their gradations 

 into larger concretions : in the schistose rocks, this homogene- 

 ous appearance of the compound body does not only, as in the 

 granitic, sometimes happen, but is their characteristic feature ; 

 and, in like manner, the composition of such schists can only 

 be discovered when they can be followed up to layers or beds 



