and Orography of the Earth's Surface. 313 



each formation furnishes us the means to know the depth of 

 each sea upon its shores, as well as at a distance from them, 

 notwithstanding that certain places, or frequently the deep- 

 est, may have received no deposit at all ; but it must not 

 be forgotten to subtract always from the absolute height the 

 possible value of the elevation to which the whole country or 

 basin has been subjected. For that reason, we attain a much 

 more certain conclusion upon the depth of the sea, when we 

 measure the height of a formation only above that of the 

 basin in which it lies. The bed must be then horizontal, and 

 contain shells of which the animals were littoral, or lived 

 in waters of a certain depth. If the rocks are, on the con- 

 trary, only alluvial, or conglomerates without fossils, then 

 their height gives no certain indications for the depth of 

 the sea, because the bottom of the basin may have been up- 

 raised. 



4th, One should always attempt to determine by the fossils 

 if a formation was littoral or formed in lagunes of salt water, 

 or in a deep sea. Such being the case, the study of malcology 

 and actinology, or, in other words, the study of the life of 

 molluscs and zoophytes, becomes daily more important to 

 geologists. 



The Tertiary Sea appears to have been between 2000 and 

 3000 feet deep along its shores, but not more than 900 or 

 2000 in its channels and straits. The greatest heights of 

 these formations have been produced by the elevation of 

 their bottoms, or by the inversions of their beds. For ex- 

 ample, they are in Switzerland 4000 feet, in Bolivia 16,000 

 feet and the like. These limits are confirmed by the heights 

 of tertiary beds which have been deposited in Mediterranean 

 Seas on a higher level than the ocean. The greatest absolute 

 height of the bottom of these former interior seas varies from 

 a few feet to 500 in Europe at least ; but we still want in- 

 formation on this subject. 



As the Tertiary beds were deposited in basins and on 

 shores, and as such deposits did not occur in the deepest 

 places, the Tertiary period must have had depths in their 

 seas that reached from 3000 to 4000 feet. 



The chalk formation consists of marine formations in deep 



