74 CLASSIFICATION OF ROCKS. 



nately, of most difficult attainment. The true phi- 

 losophical basis consists in Time: it must be sought 

 in the order of the successive productions which con- 

 stitute the visible earth ; and here, within the range of 

 stratification, there is no difficulty. As far as the 

 strata are concerned, Nature has given us the classi- 

 fication, herself: when we see these, we also see their 

 relative dates. Were there only stratified rocks, 

 nothing would be required but what follows; and in 

 spite of all the difficulties arising from the unstratified 

 ones, it is still the basis of a natural classification. A 

 body of strata, consecutive and parallel, is a class: 

 and to find the number of classes, it is only necessary 

 to ascertain each point where a new parallelism, un- 

 coincident with the preceding, occurs. The reasons 

 why each of these is a natural class, have formerly 

 appeared; and it has also been shown that peculiar 

 appearances occur, in any one later class, at the point 

 of each change; consisting chiefly in the presence of 

 conglomerates. And thus have three classes been 

 produced out of the secondary one of the old division; 

 following the same principles, but under more ac- 

 curate observations. 



But Geology now knows what it once did not, that 

 there are rocks every where interposed among the 

 stratified ones, and, moreover, lying below, and also 

 above the whole, which are the produce of fusion and 

 intrusion, not of aqueous deposition. Thus their 

 places among these are not the criteria of their dates ; 

 and hence some other mode of classing them must be 

 adopted. That they are of different dates, is indeed 

 certain; but it is neither possible to discover what 

 these are, absolutely, nor to assign, with sufficient 

 constancy and certainty, their relations in date to the 

 stratified ones; in which, the mere fact of relative 



