Chap. VIII.] trichinopoly district — arrialoor group. 127 



inclination being towards the North-East. At its Northern and Southern 



extremities the group rests on gneiss, overlapping the older groups on 



which it rests in the interval. Its thickness is not easily ascertainable, 



but not only its low anole of dip, but also the fact 

 Thickness doubtful. , ^ 



that the gneiss bottom protrudes through the beds 



at more than one spot, more than a mile from the boundary of the group, 



warrant the inference that its thickness is nowhere very great, probably 



not exceeding 1,000 feet. 



The constituents of the Arrialoor beds were derived in part from 



„ ^ . , the granitic band of Thutchuncoorchy, in part 



Source of materials. '' ^ 



from the gneiss, and a few pebbles of yellow marl 



in the conglomerate-beds, show that the older (probably the Ootatoor 



beds) were also undergoing denudation to some extent at the time of 



its formation. As compared with the lower groups, the bedding of the 



Arrialoor group is very uniform, and the beds thick and homogeneous. 



The fauna of the Arrialoor group is one of great interest. That of 



the lower beds includes many of the commonest 

 Fauna. 



forms of the Trichinopoly fauna,^ and a few of 

 these range through a large part of the group, but they are associated 

 with a large proportion of new and peculiar forms, and, as a whole, the 

 fauna offers many striking points of contrast to those of the lower 



Eesemblance to that of S^^^P^- '^^ ^^ essentially Upper Cretaceous, and 

 ' White Chalk.' ^^ many points reminds one strongly of that of 



the white chalk of Europe, especially in the abundance of Bryozoa, 



Echinida, Brachiopoda and small Corals, and the occurrence of such 



forms as Crania and Marsupites, both of which are absent in the lower 



groups. Ammonites are common in the lower beds, chiefly of peculiar 



species, but including a few, such as A. sugata, Forbes, and A. M'antelli, 



which have passed up from the lower groups. Nautili are numerous 



both in species and individuals ; all of them peculiar to the group as 



regards our Indian formation, but including some European forms^ 



