316 Palwohydrography of the Earth's Surface. 



chalk 8000 feet ; in the tertiary 16,000 feet, and in the older 

 alluvial and actual periods 18,000 feet. This furnishes us 

 with a kind of scale of value like that given by the subsi- 

 dences in the seas during the alluvial period. I shewed 

 {Proceed. Vienna Acad., 1850-51) that this last scale, ex- 

 pressed by the numbers 5, 10, 20, 30, 40, and others, corre- 

 sponds to the number of feet of the subsidences. 



2d, The possibility and impossibility of animal life under 

 certain depths of water, conduce to the belief that the seas 

 and their shores must have had always the same depths as 

 now. Molluscs and zoophytes live, the first to a depth of 

 600 feet, the latter to a depth of about 976 or even 1000 

 feet, but their most common habitation is of far less depth. 

 For example, the ostrea lives only at the depth of from 40 

 to 60 feet. In looking over the value of sea depths at the 

 time of deposition of the various formations, we find for all 

 times a depth of the sea on its shores only of 100, 200, to 600 

 feet in value. 



3d, Between these shores and the deepest places of the sea. 

 — Our table shews that this sea depth was always more 

 than 1000 feet, and from the time of Trias till later it may 

 have measured already 3000 feet. In the meantime, in the 

 chalk and more recent periods, other values of depth may 

 have added themselves to the former, because deeper valleys 

 at the bottom of the sea shew more extensive inclined planes. 

 According to this, we find that the sea had depths in the 

 Jura period of 4000 to 5000 feet ; in the chalk period, depths 

 of 4000 to 6000 or even 8000 feet ; in the tertiary times, 

 depths of 4000 to 20,000 feet; and in the actual period, 

 depths of from 4000 to 24,000 feet. 



4th, We arrive, lastly, at the final result, that the value 

 of 1500 to 2000 feet expressed nearly the middle value of 

 the depth of the sea at all times, and that this depth must 

 have been about that of the sea, if not in the primary, in the 

 oldest geological periods. 



(7'o be continued.) 



