382 PRINCIPLES AND METHODS OF PALAEONTOLOGY. 



coiulition of things. And now suppose a well to be sunk, or an excava- 

 tion made for some purpose or other, down to the original foundation 

 of Pompeii ; the digger would pass througli three layers of volcanic 

 accumulations, separating the foundations of as many cities, diflering 

 in the style of their architecture, in their sculpture, thc'ir paintings, and 

 their utensils, and clearly showing that they belonged to three separate 

 nations. It would be quite clear, again, to the excavator, that the high- 

 est city must be the latest and last built, the lowest the earliest ; and he 

 could arrive at no other conclusion than that three several races had 

 flourished and perished, one after another, on this very spot in ancient 

 times. For how great a space of time each race had remained, and 

 what was the absolute antiquity of any one, or of the whole, he would 

 be unable to say ; but their relative antiquity — the chronology of the 

 series, would be plainly indicated by the order of their superposition. 



Exactly the same reasoning is applicable to the beds of mud and 

 sand which are now accumulating and gradually hardening into rock at 

 the bottom of our present seas. Those layers which are at present being 

 deposited, necessarily lie above those which were formed in the same 

 locality a year ago ; and these, above those of the preceding year ; 

 while, on the other hand, they will be covered up by deposits of fViture 

 years. Therefore, it follows, that if ever the present sea beds are up- 

 heaved, so that their composition may be examined, the future observer 

 will find the beds containing the remains of marine animals and plants 

 superimposed upon one another, in precisely the same order as they are 

 now being formed, the oldest at the bottom, the youngest at the top; he 

 will be furnished by their order of superposition with an accurate rela- 

 tive chvonology of the changes which are now taking jjlace ; but without 

 the introduction of other considerations, he will, of course, be unable 

 to assign the absolute period at which any bed was deposited, or the 

 time occupied in the formation of the whole. 



The antiquarian called upon to estimate the probable absolute age of 

 the oldest of the cities in the imaginary case stated above, would be 

 guided by what he knew of the time required to build cities ; by his- 

 torical evidence as to the conditions under which nations replace and 

 extirpate one another; and by physical considerations based upon a 

 knowledge of the mode and rate of the formation of volcanic accumula- 

 tions of a given thickness ; but even then, he would, probably, prefer to 

 state the minimum rather than the maximum antiquity. And so the 

 future naturalist, should he have no other light than the strata now 

 forming themselves afford, can only be guided, in his estimate of their 

 antiquity and of the period occupied in their formation, by his knowl- 

 edge of the average duration of animal life, and of the rate at which 

 sediment of a given thickness can be de])osited. He may as well as- 

 sume the remains before his eyes to be accidental " sports of nature" 

 at once, as speculate upon any other foundation. 



Just as our only means of comprehending the civil history of the 

 past is to apply to ancient times those principles which a careful study 

 of the actions and motives of our contemporaries leads us to believe are 

 of universal application to mankind, so, in endeavoring to interpret 

 the moiuiments of the ancient world of geology, we must be guided by 

 Avhat we know of the present creation ; and thus having learned what 

 li\ing creatures now exist, how they are constructed, and how their re- 

 mains are becoming imbedded in the rocks now forming, we arc ready 

 to enter upon the inquiry as to what forms of life animated the ancient 

 worlds, how they were constructed, and how their remains have heev 

 handed down from those remote ages. 



