Chap. X.] valudayur and arrtaloor groups — poisdicherry. IGl 



group ; but are distributed pretty equally through the mass] of the 

 deposit. 



It is impossible to ascertain with any certainty the total thickness of 



Thickness of Anialoor ^^^^ Arrialoor beds of Pondicherry, but, judging 

 beds doubtful. f^,^^ ^^g j^^ ^^gjg ^^ ^j^gj^, ^^.p^ wherever it is as- 



certainable, and the small extent of country occupied by their out-crop, 

 they must be very much thinner than in the district of Trichinopoly. 

 If we assume the average angle of their dip to be 2°, which is probably 

 in excess of the truth, we should have a thickness of 900 feet of beds 

 exposed to the West of the Red Hills, where they dip beneath the coarse 

 ferruginous sandstones of the Cuddalore group, but any such estimate 

 must be fallacious in the case of such beds. 



Generalizations on the Arrialoor Group. 



PalcBontological distinction of Arrialoor Group, — We have seen in the 

 foregoing pages that while the Palaeontology of the Arrialoor group 

 indicates a change of some magnitude at the close of the Trichinopoly 

 period, in the sudden disappearance of certain species and the first 

 appearance of others, there is but little evidence of any correspondincr 

 interruption in the stratigraphical sequence of the deposits, nor is 

 there any sudden change in their mineral character, such as would 

 imply important change in the physical conditions of the area. 

 There does not appear to have been any upheaval of the Trichinopoly 



Physical relation of ^'P°'^*'' ^^ *^^ int^VY^l, which, to judge from 



Arrialoor and Trichino- the indications of the fossils, must have elapsed 

 poly groups. ■■• 



at the close of the Trichinopoly period. In the 

 South the deposits of the two groups appear to pass into each other, 

 or at least no definite demarcation can be discovered in the almost 

 unfossiliferous and irregularly bedded sands which lie at the junction of 

 the groups: and further North, where both formations become more 

 irregularly bedded, the strike and dip of the two groups, with one 

 exception, is, so far as they can be ascertained, identical. At only a 



w 



