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GEOLOGICAL MEMOIRS. 



of a house, containing Nummulites of all sizes, Terebratulcp, Echinites, 

 spines of CAdarites, &c, are found sometimes lying on the macigno ; 

 sometimes inclosed in it, in which latter case the macigno exhibits 

 remarkable dislocations near its edge ; and sometimes they appear to 

 project out of it. From these apparently contradictory conditions 

 we may gather, that these rocks were not formed of debris which was 

 thrown down upon the macigno, but that they were already consoli- 

 dated when they came into collision with the masses of macigno 

 which were still soft. If the problem were not attended by mecha- 

 nical difficulties, one might assume that these blocks had travelled 

 from the high chain of limestone to the north ; but until more accu- 

 rate inquiries have thrown light on that unknown epoch, these doubts 

 cannot be solved. 



[J, C. M.] 



On the Comparison of the German Jura Formation with 

 those of France and England. By Oscar Fraas. 



[Leonhard u. Bronn's Neues Jahrbuch f. Min. u. s. w. 1850, 2 H. p. 138-185.] 



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

 each other in the different districts of the earth's surface. 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 phe- 

 nomenon 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 decision with respect to its identi- 

 fication. 



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 general, by which the same stratum must, in dif- 

 ferent 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 Gr?//}^<£te-limestones are found everywhere, from Swabia to En- 

 gland ; 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 fossils appearing 

 nowhere else, inasmuch as the Portland group is the result of con- 

 ditions which are wanting in other localities. Portland limestone is 



