RETROSPECTIVE VIEW OF CERTAIN GEOLOGICAL FACTS. 151 



In such cases, the different rocks that occur in the same geological 

 position, have been called equivalents of each other. An instance, 

 mentioned in a preceding chapter, may serve to explain what is meant 

 by a geological equivalent. In the beds of transition limestone at 

 Llanymynah, which are very regularly stratified, one stratum of the 

 best limestone suddenly terminates, and its place is supplied by a 

 bed of marie of equal thickness; in the same manner as we might 

 suppose part of a course of bricks to be taken out of a wall, and its 

 place filled up with clay ; the clay would be the equivalent of the 

 course of bricks. 



In many of the lower conformable rocks, there is a tendency to 

 reproduction in the upper parts of the series ; thus, though the regu- 

 lar order of succession may be granite, gneiss, mica-slate, and slate 

 (the clay-slate of Werner), w^e often find beds of granite among 

 gneiss and mica-slate, and sometimes even in slate. When, however, 

 we consider, that the chemical composition of all these rocks is very 

 nearly the same ; that silex forms on the average three fourths of 

 their constituent parts, and alumine about one sixth or one eighth, — 

 the proportions of the remaining parts cannot greatly affect the condi- 

 tion of the mass ; and it is to the circumstances (whatever they may 

 be) which have occasioned a more or less rapid consolidation of the 

 parts, that we ought, probably, to attribute the formation of granite in 

 one part of a mountain, and of gneiss, mica-slate, or slate in another, 

 and the re-appearance of granite above the latter rocks. An enquiry 

 naturally suggests itself, on observing that the order of succession in 

 rocks is not invariably the same in distant countries. Are the simi- 

 lar rock formations in distant parts of the world contemporaneous ? 

 or were rocks of different classes forming at the same period ? Is the 

 granite of England, for instance, more or less ancient than the gran- 

 ite of the Alps? Or, are the secondary strata of one country as old 

 as the primitive rocks of another ? 



Were it not for the organic remains in different rocks, we could 

 not (as Cuvier has well observed) be certain that all rock formations 

 were not contemporaneous. With respect to those rocks which con- 

 tain no organic remains, and under which there are no other beds 

 containing organic remains, we cannot ascertain whether they were 

 contemporaneous, or formed at different and distant epochs. The 

 beds of granite which are nearly vertical in mountain ranges, must 

 have acquired a considerable degree of solidity, before the period 

 when the beds were raised : but if we date their age from the epoch 

 of their elevation, we shall be obliged to admit the different ages of 

 granite mountains, and that the granite of Charnwood Forest is more 

 ancient than that of the Alps. Of this we have as direct proof as 

 we could possibly require. In the Alps, the beds of the upper se- 

 condary strata, analogous to our magnesian limestone, lias, and oolite, 

 where they approach the central granitic range, are raised into nearly 

 a vertical position conformable to that of the beds of granite, and 



