

TIIK .U'KASSK' SYSTKM 455 



.1.- \\rll its other ]n'culi:ir species, such as Phylloceras ptychoicum and 

 IVaagenia hybonota. In the Strainberg limestone Phylloceras 

 silesiacum and Perisphinctes transitionis also occur, and this 

 limestone may be tin- i-i|iiivalent of our Purbeckian. This t'acies 

 extends from the Central Alps and the north of Italy through tin- 

 Tyrol ami Austria to the Carpathians and is found also in the 

 Balkan States. 



4. The Russian Fades 



As already stated the oldest Jurassic deposits in Russia are of 

 Oxfordian age, and the fauna of these and the succeeding Corallian 

 and Kimeridgian Beds is similar to that of their equivalents in 

 Western and Central Europe ; the highest part of the series forms 

 a special Russian or Boreal facies, differing in so many respects 

 from the normal Portlandian that Nikitin proposed the name 

 Vulyian for this set of beds. Although the thickness of the group 

 is very small only a few feet of sand and shale with layers of 

 phosphatic nodules several Ammonite zones have been detected in 

 it, and it passes up into beds of Lower Cretaceous age. 



The beds were evidently formed in shallow water and under the 

 influence of strong currents. The prevalent genera of Ammonites 

 are Perisphinctes, Virgatites, Holcostephanus, and Craspedites, with 

 Belemnites of the absolutus type and shells of the bivalve Aucella. 



The best sections are near Moscow and in the province of 

 Simbirsk, especially between Simbirsk and Syrzan on the Volga, 

 where, however, the beds are very thin, the succession according to 

 Professor Pavlow being as follows in descending order : 



4. Calcareous sands with Craspedites subditus .... ? 6 feet 

 3. Glauconitic sand with Holcostephanus Bladei, Hole, triplicates, 



and Perisphinctes cf. yiyus . . . . . . . 1 ,, 



2. Sand with phosphate nodules and shales with Virgatites 



virgatus, Belemnites absolutus, etc. . . . . . 3 ,, 



1. Shales with Perisj)hinctes Bleicheri and Aucella Pallasi . '1 ,, 



The subditus zone is correlated by Pavlow with the Purbeck 

 Beds, but this is uncertain (see p. 491). It will be noticed that 

 the underlying Portlandian is only 6 feet thick, but near Moscow 

 the corresponding beds have a thickness of 26 feet 



HISTORY OP THE JURASSIC PERIOD 



Liassic Time. We have seen tliat it is convenient to date 

 the beginning of this period from the subsidence which permitted 

 the Southern Sea to make its way into the great Inland Sea which 



