250 GEOGRAPHICAL ABTD GEOLOGICAL DISTRIBUTION. 



determination of the question. Yet the recession southward of reef- 

 structures, in the making of which the modern type corals were 

 largely involved, during the succeeding periods of the Mesozoic 

 and Cainozoic eras, seems to indicate that this must have been the 

 case; for as far as the evidence from these later structures goes 

 there can "be no reasonable doubt as to a progressive lowering of the 

 oceanic temperature correlatively with an advance in time. And if 

 this was the case from the Triassic period onward, there is every 

 reason to suppose that it has been so from a much earlier period. 



Reef-structures appear to have been very extensively developed 

 in South-Central Europe during the Triassic period, and not un- 

 likely much of the giant dolomites of the Tyrol, whose abrupt and 

 pinnacled masses so wonderfully diversify the face of the country, 

 is the product of the unceasing labours of the minute polyp. In the 

 succeeding Jurassic period a more or less continuous coral sea oc- 

 cupied a considerable portion of Western and Central Europe, as is 

 evidenced by the vestiges of reefs which still remain in England, 

 France, Germany, and Switzerland. During the deposition of the 

 Oolites the reef-structures appear to have attained their maximum 

 development, the shallow coral sea, with its atolls and barrier reefs, 

 extending as far east as the Carpathian Mountains, and covering 

 much of the region now occupied by the Alps. The British area 

 was still favourable to the growth of the coral polyp. With the 

 beginning of the Cretaceous period there would seem to have been 

 a gradual deepening of the oceanic bottom to the north, and the 

 introduction of conditions inimical to the proper development of 

 coral life, for the number and extent of the reefs occurring there 

 are comparatively very limited ; a Southern European belt, on the 

 other hand, is very coralliferous. No doubt the gradual lowering 

 of the oceanic temperature had much to do with this recession, 

 more, in fact, than the simple lowering of the oceanic bottom, 

 since the latter, if gradual, while it would almost certainly check 

 a lateral extension of coral structures, would scarcely tend towards 

 their complete or wholesale obliteration. The coral formations 

 would still persist with the conversion of connected land-masses 

 into islands, and after the complete submersion of these last, just 

 as we now find them over the deeper oceanic abysses. That the 

 temperature at this period was not entirely too low, however, in 

 all parts of the region under consideration, is proved by the Upper 



