PART II. CHAPTER XIII. 161 



Tests of the different Ages of Aqueous Roctfs. 



we examine strata in the direction of their planes, although by 

 no means for indefinite distances. This might have been ex- 

 pected ; for although many species of animals and plants have 

 a wide geographical range, yet each species generally inhabits a 

 small part only of the entire globe, and is often incapable of 

 existing in other regions. But, in those cases where the fossils 

 vary, the mineral character of the rock often remains constant ; 

 and, on the other hand, the fossils are sometimes uniform through- 

 out spaces where the lithological nature of the rock is variable. 

 In this manner we are frequently enabled to prove the contem- 

 poraneous origin of the same formation by one test, when the 

 other fails. 



Secondly, while the same fossils prevail in a particular set of 

 strata for hundreds of miles in a horizontal direction, we seldom 

 meet with the same remains for many fathoms, and scarcely 

 ever for several hundred yards, in a vertical line, or a line trans- 

 verse to the strata. This fact has now been verified in almost 

 all parts of the globe, and has led to a conviction, that at suc- 

 cessive periods of the past, the same area of land and water has 

 been inhabited by species of animals and plants as distinct as 

 those which now people the antipodes, or which now coexist in 

 the arctic, temperate, and tropical zones. It appears, that from 

 the remotest periods there has been ever a coming in of new 

 organic forms, and an extinction of those which pre-existed on 

 the earth ; some species having endured for a longer, others for 

 a shorter time ; but none having ever re-appeared after once dy- 

 ing out. The law which has governed the creation and extinc- 

 tion of species seems to be expressed in the verse of the poet, 



Natura il fece e poi ruppe la starapa. Ariosto. 

 Nature made it, and then broke the die. 



And this circumstance it is, which confers on fossils their highest 

 value as chronological tests, giving to each of them, in the eyes 

 of the geologist, that authority which belongs to contemporary 

 medals in history. 



The same cannot be said of each peculiar variety of rock ; 

 for some of these, as red marl and red sandstone, for example, 

 may occur at once at the top, bottom, and middle of the entire 

 sedimentary series ; exhibiting in each position so perfect an 

 identity of mineral aspect as to be undistinguishable. Such 

 exact "repetitions, however, of the same mixtures of sediment 

 have not often occurred, at distant periods, in precisely the same 

 parts of the globe ; and even where this has happened, we may 

 usually avoid confounding together the monuments of remote 

 eras, by the aid of fossils and relative position. 



