388 PROPOSED MODIFICATION [Ch. XVL 



by deposits, mixed with organic remains ; the bottom of the 

 sea by the accumulation of gravel, sand, and marine pro- 

 ductions. Since, therefore, these various occurrences, in each 

 geological epoch, are contemporaneous, it is evident that 

 vegetable, lacustrine, marine, and other formations may be 

 the equivalents of each other, though greatly differing in 

 nature : and, as one and the same district may exhibit all 

 these formations, even resting on the same rock or diluvial 

 deposit, so each of these, confined to an extensive tract or 

 country, owing to its being entirely situated above or below 

 the sea-level, would be parallel or equivalent to the others, 

 although one would be characterised by terrestrial, another 

 by lacustrine, and the third by marine organic remains. 



This is a most important consideration, for it teaches us 

 that we must not only become acquainted with the order of 

 succession, in whichjhe various families of plants and animals 

 appear to have been called into existence, but we must also 

 determine the precise genera and species of land and of fresh 

 and salt water animals and vegetables, which flourished 

 during the same geological epoch. 



The frequent intermixture of all kinds of deposits, in the 

 same group, will, perhaps, ultimately lead to the full deve- 

 lopement of organic equivalents ; and then the uncertainty 

 concerning the analogy of remote series of rocks, which at 

 present perplexes geologists, will be greatly diminished. 

 This task, however, cannot be speedily accomplished; indeed, 

 even the animated beings peculiar to every climate, and to 

 various situations in the same country, during the modern or 

 present epoch, have not as yet been ascertained. 



As regards the igneous or volcanic power, the remaining 

 cause of changes now in progress, we may presume that it 

 has been active, from the period of the formation of the 

 primary rocks to the present time. It is very possible that, 

 during the consolidation of these rocks, dykes and veins may 

 have been formed, like those of lava in the cones of volcanos. 

 But the cases appear to be very dissimilar ; one of these is the 



