106 Mr Oscar Fraas' Observations on the Effects 



Observations on the Effects of Local Influences upon the For- 

 mation of Strata. By Mr OscAR Fraas. 



Mr Oscar Fraas, in his interesting memoir '* On the Com- 

 parison of the German Jura Formation with those of France 

 and England," introduces his subject with the following 

 observations on the effect of local influences upon the for- 

 mation of strata, which will interest our geological readers : — 

 " The older the formations are, the easier is their comparison 

 with each other in the different districts of the earth's sur- 

 face. Because the further we trace the genesis of the earth 

 in its earlier times, the more uniform do we find the soil and 

 climate with its inhabitants ; so that species exist in the 

 transition rocks which are identical in Asia, Europe, and 

 America. In the newer formations this phenomenon never 

 re-occurs, because the younger the earth becomes, the more 

 manifold are the conditions of soil and climate. In the 

 Jurassic period, then, soil and climate have already become 

 so different, that it is only by a comparison of the same 

 stratum in different countries, that we can arrive at a deci- 

 sion with respect to its identification. 



The dissimilarity of the bottom of the sea, and that of the 

 bays and gulfs, exerted an influence upon the formation of 

 strata too great to admit of an extensive deposit appearing 

 everywhere the same. Besides, there were the conditions of 

 the shores, the propinquity or distance of the land, the depth 

 of the sea, the mouths of rivers, and local influences in gene- 

 ral, by which the term stratum must, in different parts, have 

 been differently formed. The younger the Jurassic strata 

 are, consequently the more apparent become the different 

 local conditions of a stratum. Whilst the deposits forming 

 the "black Jurassic" rocks are pretty similar in different 

 countries, they already differ, in a greater degree, in the 

 " brown Jurassic" series, and in the *' white Jurassic" rocks 

 the characters are so manifold, that it is impossible to prove 

 the identity of certain strata. The Arietes and Gryphace 

 limestones are found everywhere, from Swabia to England ; 



