GEOLOGY OF ARCTIC EURASIA 8 1 



As no Devonian is known on the top of the horsts, only between 

 them, and as the youngest marine strata on the surface of the horsts 

 are Silurian, the deformative agencies that disintegrated the old 

 continent must be referred to the end of the Silurian, or to early 

 Devonian, more probably to the latter. 



The Geological History of the Marine Deposits 



Since that time the stratigraphic history of Arctic Eurasia has been 

 recorded chiefly in the seas between the horsts. Different trans- 

 gressions, although not always known in detail, appear to have 

 followed the direction of the Urals in Europe and probably of Ver- 

 khoyansk Ridge in Asia. The communication of corresponding Amer- 

 ican and Eurasian Upper Paleozoic and Mesozoic seas through the 

 Arctic and the migration of their faunas were guided by these well- 

 expressed geosynclines. These great primary structural features also 

 explain the connection between Arctic and Mediterranean faunas. 

 The Paleozoic strata near the horsts, or positive elements, are always 

 folded, partly owing to the resistance they offered as unmovable 

 masses, partly because of continuing sinking within the graben and 

 especially in the major depressions, or geosynclines. 



A strong diastrophism or crustal deformation again took place 

 between the Upper Paleozoic and the Lower Mesozoic, with its 

 maximum at the Upper Jurassic. It is quite possible that the dis- 

 membering of old Arctic Eurasia into horsts and grabens referred to 

 above not only had taken place during the Lower Paleozoic but 

 continued in the Mesozoic as well. 



The Mesozoic diastrophism separated the present Taimyr Penin- 

 sula from the Central Siberian Plateau with a broad sunken strait 

 which existed till the latest post-Pliocene. Mesozoic strata (Triassic, 

 Jurassic, Cretaceous), widely distributed in northern Asia, very 

 distinctly indicate and delineate tracts sunken in the Mesozoic. In 

 the Verkhoyansk Ridge they are folded, but otherwise they are here 

 but little disturbed. Concerning this ridge it is necessary to notice 

 that its folding probably has been continued till very recent time, 

 geologically speaking. 



Near the Ural Mountains an extensive Mesozoic transgression, or 

 invasion of the sea, followed a regional depression now marked by 

 Mesozoic deposits. At the eastern end of Arctic Eurasia Mesozoic 

 sediments of the same type are known in the Anadyr region. 



The Upper Jurassic faunas of the Arctic belong to the Boreal, or 

 Russian, type, the distinction being dependent, in the opinion of 

 some students, upon climatic differences of the period. The Lower 

 Cretaceous faunas, immediately following the Jurassic, have also 

 a Boreal character. 



