10 PICTURESQUE SKETCHES 



it is meant that certain groups of species are chiefly 

 met with in rocks of one age, the various species of 

 each group being more rarely found in those of the 

 preceding and succeeding periods, but never met with 

 again in abundance. And this is not the less true 

 because a few of them, escaping it would seem as if 

 by some unusual powers of endurance, and stragglers 

 from the general herd, have continued to exist for a 

 long time, and through many subsequent periods. 



From these two positions, both of which have 

 been satisfactorily proved in the progress of Geology, 

 namely, that fossils are characteristic of formations, 

 and that in all the different parts of animal and 

 vegetable structure there is reference to every peculi- 

 arity of habit in the complete organized being; it 

 follows that the study of fossils becomes an impor- 

 tant and necessary branch of Geological research. 



There are, however, two ways, by means of each 

 of which satisfactory conclusions have been arrived at 

 from the study of organic remains; and as, of these 

 two, one chiefly bears upon Geology, while the other has 

 reference quite as much to Natural History generally, 

 so both unite in laying the foundation, and building 

 the superstructure, of that general history of the 

 world which it is the true object of Geology to describe. 



It is not difficult to explain the value of fossils in 

 each of these two respects. To the Geologist they 

 are of value, not only in the identification of strata 

 in different parts of the same district when the 

 mineral character of the beds is doubtful or variable, 

 but also in determining those groups of strata which 

 shall be either classed together as having something 

 in common, or separated as entirely distinct. Viewed 



