HOMOTAXIAL EQUIVALENTS OF THE PERMIAN 329 



rocks of Russia extend from the Arctic Ocean to the Black 

 Sea — a distance of 1500 miles — and from the Urals westward a 

 distance of 1200 miles. In the central and southern parts of 

 the area is a thin covering of Cretaceous and Tertiary deposits. 

 This vast basin, with its nearly horizontal strata, is comparable 

 to our own Carboniferous basin of the Mississippi valley. In 

 the latter region the lower portion of the sequence — or Coal 

 Measures — predominates. In Russia the upper part, or Permian, 

 forms the surface in most of the region. 



Around the margins of the great basin, especially on the 

 west and east sides, the Carboniferous is well developed. The 

 lower Carboniferous, made up of limestones, is well displayed, 

 lying immediately upon the Devonian. Relatively speaking, 

 the Coal Measures are not very well represented, though the 

 southern coal field, or Donetz basin, covers 1200 to 1500 square 

 miles, and the central field, or Toula basin, has about the same 

 area. On the flanks of the Urals some coal is also found. 



The most typical sections of the Permian are in the Kama 

 River Valley. The great Volga Valley, above Samara, is occupied 

 chiefly by the so-called Permo Trias. 



Nature of the rocks. — The beds that are called Permo-Carbon- 

 iferous, Permian, and Permo-Trias, which occur in the Kama River 

 Valley present the most typical phases of Murchison's " system.'.' 

 The whole succession is tripartite. Shales, sandstones, and 

 marls are separated medially by heavy dolomitic limestones. 



The lower member consists of argillaceous and sandy shales, 

 shaly sandstones, marls, and some impure limestones. Some- 

 times conglomerates are present. Abundant fossils are repre- 

 sented. Upwards of 300 species have been listed. A distinctive 

 flora is also present. 



The median terrane is made up chiefly of massive dolomitic 

 limestones, separated by calcareous shales. It forms a striking 

 contrast to the beds above and below. 



The upper member is formed of variegated argillaceous and 

 sandy shales, brown shalv sandstones, some of which are copper- 

 bearing, marls, and occasionally thin limestone bands. Gypsum 



