﻿FRAAS ON THE JURA FORMATION. 



4 'A 



neither found in Swabia nor in Switzerland ; but there are synchro- 

 nous deposits which may be paralleled with it. In general, therefore, 

 in speaking of a comparison of the Jurassic rocks, we have less to say 

 of strata of the same character than of the same time, and we are con- 

 fined to the synchronism of the formation. Therefore the task of 

 comparing the Jurassic rocks of different localities infers the com- 

 parison of synchronous deposits with each other, and the consider- 

 ation of their local differences and conditions, as well as the species of 

 animals and plants contained in them. The result of this examina- 

 tion will be a restoration of the form of the sea at the Jurassic epoch, 

 its extension and boundaries, together with its inhabitants. In order 

 to accomplish this, the observation of the local influences upon the 

 formation of the strata is of the greatest importance. We have there- 

 fore to observe : — 



1 . The influence of the shore ; — its condition, and its proximity 

 or distance [to the place where deposition is going on\. — The Lias 

 sandstone, for instance, which is strongly developed in Swabia in the 

 lower " black Jurassic " series, is never found after leaving the Jura, 

 until we reach northern England, in the form in which it occurs with 

 us. There it reappears, because the causes of its development are the 

 same as in Swabia. The Lias sandstone is nothing else than the de- 

 position of the Lias together with the Keuper, — the transition of the 

 one formation into the other. The yellow upper Keuper sandstone, 

 which is itself merely a continued deposit of Keuper sandstone, is the 

 first condition. and factor of the Lias sandstone. The circumstance of 

 sandstone being formed, generally speaking, proves the presence of a 

 shore consisting of sandstone ; which, loosened in the water, gave rise 

 to new precipitations in another form. The sandstone shore in Swabia 

 was the Black Forest, in the East of France the Vosges. The further 

 we go away from these sandstone shores, the more destitute is the Lias 

 of the sandstone formations. Thus also were formed similar strata 

 of the lower " black Jurassic" formation, which appears as an alter- 

 nation of lime- and sand-stone in proximity to the shore ; and far 

 from the shore, in the depths of the sea, as a pure calcareous deposit. 

 Normandy is a normal type for pure pelagic precipitations, — forma- 

 tions far from the shore in the open sea. Here sandstone is not at 

 all known in the Jurassic series. The thickness of the strata is quite 

 insignificant, because accumulations are not possible in the open sea. 

 In this Lias, saurians, fish-teeth, and breccia (which prove the prox- 

 imity of a shore) are either not found at all, or are exceedingly rare. 

 At Fontaine Etoupefour (Depart. Calvados) the whole Lias is from 6' 

 to 8' thick, but even in this thickness all the divisions of the Lias are 

 represented in miniature ; — a proof that the thickness also of the 

 strata, and not their materials only, depend upon the proximity or 

 distance of the shore. 



2. The depth or shallowness of the sea exercises an influence upon 

 the conditions of its inhabitants, and consequently upon the species of 

 the imbedded organic remains. — We seek in vain in the Swabian Lias 

 for corals, which are especially evidence of shallow seas : Calvados is 

 very rich in them . In Burgundy, the J ura, and Normandy, whole banks 



