of Local Influences upon the Formation of Strata, 107 



they commence the Jura formation. But those which form 

 the last member of the series, the Portland limestones, are 

 only found in Dorsetshire ; identically similar rocks and fos- 

 sils appearing nowhere else, inasmuch as the Portland group 

 is the result of conditions which are wanting in other locali- 

 ties. Portland limestone is neither found in Swabia nor in 

 Switzerland ; but there are synchronous deposits which may 

 be parallel 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 confined to the synchronism of the formation. There- 

 fore the task of comparing the Jurassic rocks of different 

 localities, infers the comparison of synchronous deposits with 

 each other, and the consideration of their local differences 

 and conditions, as well as the species of animals and plants 

 contained in them. The result of this examination 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 influ- 

 ences upon the formation of the strata is of the greatest 

 importance. We have, therefore, to observe — 



1. The influence of the shore, its condition, and its proxi- 

 mity 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 Eng- 

 land, in the form in which it occurs with us. There it re-ap- 

 pears, because the causes of its development are the same as 

 in Swabia. The lias sandstone is nothing else than the depo- 

 sition 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 sand- 

 stone. The circum stance of sandstone being formed, generally 

 speaking, proves the presence of a shore consisting of sand- 

 stone, which, loosened in the water, gave rise to new preci- 

 pitations 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 



