Vol. 52.] STBATA BETWEEN THE KIMERIDGIAN AND APTIAN. 553 



first traces of a new ' hydrocratic ' displacement of the shore-lines, 

 by which a further part of the Purbeck continent disappeared and 

 •communication with the southern sea was opened. Thus were the 

 first pioneers of the southern fauna permitted access to the Northern 

 basin ; and in the succeeding zone of Hoplites regalis this fauna 

 took possession of the region and supplanted the Northern species. 

 There still remained, however, a considerable portion of the pre- 

 existing continent, and this formed the Wealden land-area. Now 

 let us consider what had happened meanwhile in the Eastern 

 Kussian part of our region. 



As the Table (facing p. 548) shows, the Petchorian beds are over- 

 lain in Russia by the Upper Neocomian (Simbirskian) beds with 

 Simbirslcites versicolor, discofalcatus, Decheni, etc., and in no part of 

 Russia are the beds with Hoplites regalis known. We may conclude 

 from this that by the time when the ' hydrocratic' movement, which 

 began in Central Russia, had reached the western portion of Europe 

 and had opened there a communication between the northern and 

 southern basins, Central Russia itself had once more become dry 

 land, and remained so until Upper Neocomian times, at which 

 period, in consequence of renewed ' hydrocratic ' movements, an 

 extensive sea again covered North-eastern, Eastern, and in part 

 Central Russia. In England and probably in Germany, during 

 the same Upper Neocomian epoch, a boreal fauna regained the 

 advantage over the southern invaders, and beds with SimbirsJcites 

 speetonemis, Decheni, subinversus, etc., were deposited. It is, more- 

 over, interesting to note that precisely at this period a few forms 

 of SimbirsJcites — the characteristic representatives of the boreal 

 fauna — penetrate far to the south and appear amidst the southern 

 fauna in the Crimea and even in Southern France. During the 

 succeeding stages of the Aptian and of the Gault a common fauna 

 of mixed character establishes itself widely, but in Western Europe 

 we observe in it the predominance of southern forms, such as 

 Acanthoceras Martini, Belemnites minimus, and other allied species, 

 results due probably to a new ' geocratic' displacement of the shore- 

 lines in Central Russia, and to the rising of Aptian dry land, on 

 which the plant-bearing sands and sandstones of the Sparrow Hills, of 

 Tatarowo, and of Klin (government of Moscow) were accumulated. 



From this sketch it becomes apparent that the 'hydrocratic' and 

 ' geocratic ' movements of the shore-lines did not take place simulta- 

 neously over the whole of this vast area, but that each movement 

 passed slowly through the region latitudinally. And it is this 

 great march of movements — whether of solid crust or of sea-level it 

 matters not — that has brought about the complicated sequence of 

 the different faunas in the different parts of the region above 

 described. Whence it follows that only by taking into consideration 

 the course of events over the whole of the region can we hope to 

 obtain a general and intelligible picture of this particular chapter 

 of geological history in any country which formed part of it. It 

 is, however, only on the results of detailed local studies that any 

 successful attempt to classify the events must be based. 



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