314 M. Ami Boue on the Palozohy Urography 



water, and of a littoral one. The seas during that time must 

 have been very similar to the tertiary period, having a depth 

 of from 600 to 800 feet ; but the chalk itself was deposited 

 in water of a depth varying from 1200 or 1300 to 3000 feet. 



The Jura Sea, or at least that part of it where the Jurassic 

 beds were formed, was a sea of more than 3000 feet in depth. 

 The littoral deposits of the seas were formed under a sea of 

 1300 or 1500 feet in depth, — the coral rag, similar shallow 

 sea. In Western Europe we find for it the latter, a sea of 

 nearly 800 feet deep, or even less. 



The Trias formation shews by its thickness that its sea 

 had a depth of at least 3000 feet, with occasional places 

 more shallow. 



The Zechstein and the red secondary sandstone were de- 

 posited upon shores in water less than 1000 feet deep. This 

 is confirmed by coralline formations and plutonic eruptions. 



In the primary periods the seas were deep and shallow ; 

 the deep about 2000 or 3000 feet in depth, the shallow indi- 

 cated by the many coral deposits found in the formations. 



In taking a general glance of the value of these depths in 

 the various periods of time, we learn that the marine forma- 

 tion never covered the whole sea bottom. Besides the de- 

 posits on shores and in sea channels or straits, and from 

 currents that now accumulate in our deeper seas, we have a 

 great part of the sea bottom that remains now as formerly 

 uncovered by aqueous deposits. It is difficult to calculate 

 the present extent of plutonic eruption that takes place ; but 

 it appears that volcanic matters are accumulating now in 

 deep seas, as must have been the case formerly. But this 

 will seldom occur in the deepest places where the pressure 

 is very great. 



These values of the deep and deepest places can be esta- 

 blished still for each period, if we admit that the scale of 

 subsidences from the older times to the newer is an ascend- 

 ing one like that of elevations and vaultings. In this case 

 we can use the height which some formations attain through 

 elevation ; by which events the exact time is given by the 

 geognostical relations of position. The inclination or immer- 

 sion of beds, and the repetition of elevation on the same spots, 



