Chap. II.] general description of country. 25 



Egerton and M. D'Orbigny attach to the Tertiary and Upper Cretaceous 



aspect of the ichthyous portion of the fauna, that in spite of the 



counter-evidence afforded by the Neocomian forms of the Cephalopoda, 



they were incHned to assign the Pondicherry beds to a period not earlier 



than the Upper greensand or Gault ; considering that, while, on the one 



hand, a number of Neocomian species had lived on to a later period 



than their nearly related congeners of Europe ; on the other hand, 



certain tertiary genera had begun to appear at an earlier period in these 



tropical seas, to which, at the present day, some of them are exclusively 



confined. That no one surmised the true state of 

 Explanation. 



the case, viz. that the fosslis were obtained from 



two different deposits, is probably due to the resemblance of the two 



rocks in which the fossils occur, and the knowledge that they were all 



obtained from within an area of not more than two or three square miles 



in extent. Indeed, I myself, who had visited the locality, and was aware 



of the anomalous character of the fauna, had no suspicion of the 



existence of two sets of beds until the Geology of the Trichinopoly 



district being worked out, I found that several of the Pondicherry 



species were especially characteristic of the highest or Arrialoor 



group, while of the Ammonitoid Cephalopoda so abundant in the 



Pondicherry beds, and Neocomian in their affinities, but one or two 



species occurred in any part of the Trichinopoly series, and these few 



only in the Ootatoor or lowest group. This led me to examine the 



association of species in specimens of the Pondicherry rocks, and the 



result strongly confi.rmed the supposition of the existence of two distinct 



faunas : a supposition which re-examination of the beds at length finally 



established. 



A detailed description of the locality will be given further on, and it 



will be sufficient to remark here, that the Arri- 

 Extent of Cretaceous rocks. 



aloor beds occupy a narrow band of country to 



the West of the Red Hills, cropping out from beneath the uufossiliferous 



D 



