and Orography of the Earttis Surface. 31 5 



do not render this more difficult, because the same took place 

 for the subsidences. 



In the old alluvial and recent tertiary periods, the great 

 Sea Depths and their middle depths were nearly those of our 

 present seas, which we can prove especially by the height of 

 volcanic mountains and chains. In the tertiary times the 

 elevations of the chalk and eocene formations indicate sea 

 depths of 8000, 9000, 10,000, to 24,000 feet, which was the 

 case immediately after the chalk period. The middle sea 

 depth of the ocean may have been then from 4000 to 5000 feet. 



In the chalk period) the height of the elevated Jurassic 

 beds indicates seas of from 6000 to 11,000 feet, and probably 

 still more in depth when we consider the Himalaya. Their 

 middle depth may have varied between 2000 and 3000 feet. In 

 the Jura period the heights of the elevated Trias seem not 

 favourable to the existence of sea depths of that greatness. 

 The middle depth may probably have been 3500 feet, and the 

 deepest places may have measured 5000 to 6000 feet. 



In the Trias period the well-known elevations of old forma- 

 tions, as well as the great plutonic deposits, appear to indi- 

 cate for the deepest places of the seas between 4000 and 5000 

 feet, and the middle depth may not be far from 2500 feet. 

 The greatest known height of the Trias exists in Bolivia, 

 where it is preserved still partly on both sides of the eastern 

 Cordillera, and reaches sometimes, according to D'Orbigny, 

 the height of 2000 feet (Compt. R. Acad. d. Sc, Paris, 1843, 

 v. 17, p. 388), a circumstance only to be explained by elevation. 



Lastly, in olden times the sea may have had no deep 

 places, and only a middle depth of from 2000 to 3000 feet ; 

 for all the high summits of older rocks are only the conse- 

 quence of later elevations, but, on the contrary, all the rest 

 of the oldest islands or continents do present themselves only 

 as very low hills or even plains. 



When we construct a table of the values of the probable 

 depths of the sea at different times on its shores, as well as in 

 the middle, we come to the following interesting results : — 



1st, When the deepest places of the primary sea were about 

 2000 to 3000 feet, the middle value of the deepest places in 

 the Trias and Jura periods was about 4000 feet ; in the 



