THE JURASSIC AGE. 



or earliest, and proceeding upwards to the uppermost or latest. 

 It may, nevertheless, be useful to indicate the names given to 

 them in the works of M. D'Orbigny and other French geologists, 

 annexing the estimated average thickness of the stages. 



These approximate estimates of mean thickness are much below 

 the real measures on the slopes of the Alps, the western declivity 

 of the Yosges, and through all the series from Avallon to 

 Tonnerre. 



321. In fig. 152 is presented a section of the Triassic and 

 Jurassic formations extending from the Yosges to Sommevoire 

 (Haute Marne), directed towards Paris,* where the series of strata 

 is complete, the successive stages of each formation being marked 

 with the Roman numerals. 



322. From an elaborate examination of the strata composing 

 these ten stages in various countries and in different parts of the 

 globe, it appears, that during the Jurassic age, the earth was 

 subject to all the physical vicissitudes which are at present inci- 

 dental to it. During each of the ten periods there were conti- 

 nents and oceans. As at present, peculiar classes of marine 

 animals inhabited the coasts, the deeper littoral parts, and the 

 still greater depths of the ocean. Then, as now, there were sub- 

 marine currents, waveworn shores, sheltered gulfs and tranquil 

 bays. The condition of the strata proves that during this age slow 

 undulations of the crust, such as those which are now manifested 

 in the Scandinavian peninsula (118), were frequent. This is espe- 

 cially manifested by the state of the strata composing the 3rd, 

 4th, 5th, 6th, and 7th stages, at Chaudon, in the department of 

 the Basses Alpes in France. 



323. The Jurassic age is distinguished zoologically from the 

 Triassic age by the absence of forty-two genera, which became 



* For the continuation of this section to Paris, see 



i 2 



396 and 456. 

 67 



