134 CRETACEOUS ROCKS OF S. INDIA. [pART II. § 1. 



which caps the high ground to the West of Amaloor. Again similar heds, associated with 

 Extent of white sands greyish and greenish sands, and soft ochreous shales, constitute 



Northward. ^^tQ lower part of the group to the East of Kurribiem, beyond 



which nothing is visible for some miles. They are generally unfossiliferous and have little 



perceptible dip. Where ascertainable it does not exceed 3° or 4°. 



In the bottom beds of the group are intercalated calcareous bands, similar to those of the 



.„, , , ,. Trichinopolv heds immediately below them. Indeed, it is fre- 



Difficulty of separating j-ii^-i^iu^i^v^ j ^ 



groups. quently almost impossible to determine the exact limits of the 



groups ; as their beds are locally confonxiable, and only fossiliferous in a few places. Here and 

 there the point can be determined by the discovery of some characteristic forai of fossU, but 

 in general the boundary given in the map can only be considered as approximately accurate. 



I have already mentioned, that in the Arrialoor, as in the lower groups, the general strike 

 of the bedding is more Northerly than the boimdary of the group, or in other words, the deposits 

 in the Southern part of the area cover a wider extent of countiy than an equivalent series in the 

 North of the district.* Before therefore proceeding to describe the lower beds of the groups in 

 the Northern part of the district, it will be desirable, following the order I have hitherto observ- 

 ed, to return to the beds in the neighbourhood of Arrialoor, and to trace them Northwards as 

 part of the lower fossiliferous zone to their disappearance beneath the alluviimi of the Vellaur. 



The broad band of cotton soil and alluvium that extends up the valley of the Murdayaur, 

 to within a mile of Arrialoor, entirely conceals the underlying Cretaceous rocks. North of this 

 alluvium, a narrow belt of Cretaceous rocks is exposed, he- 

 Beds s. E. of An-ialoor. ^^^^^^ ^^^ former and the capping of Cuddalore sandstones, 

 which overlies the Cretaceous rocks in the Eastern part of the district, but these rocks, which 

 consist chiefly of white sands, belong rather to the middle unfossiliferous zone, the description 

 of which may be deferred for the present. 



The rocks around Arrialoor contain a fauna very similar to that of the lower beds, except that 

 the Trichinopoly species, which form a considerable proportion of the latter, haA'e somewhat 

 diminished in number. The beds are well seen at the Kalingiila, or waste channel, of the 

 large tank to the West of the native town. At this spot they are nearly horizontal, dipping 

 only 2" to the Eastward, they consist of calcareous sandstone and grit, somewhat conglomeratic, 

 and abounding in fossils ; among which Nautili of four or five species are very abundant, 

 Fossiliferous beds of Fecten 5-costatus, Area Trichinopolitensis, Area abrupta and 

 Arrialoor. Cardium HiUanum also occur together with species of Baeulites, 



JVatica, Turritella, Pleurotomaria, Spondylus, Plicatula, Ostrea, Vulsella ? Peetunculus, 



* I am not in a position to assert that the beds in the South are thicker than in the North, 

 because I have no reason to believe that any calculation based upon the amount of their dip 

 would give reliable results. I have endeavoured to prove in the case of the Ootatoor and 

 Trichinopoly groups that there is strong reason to believe their bedded structure due to bank- 

 ing of the deposits rather than to equable horizontal deposition, and we have also seen in the 

 Southern part of the Arrialoor Group, at all events there are evidences of similar irregularities 

 of acciimulations. I think it very probable that the greater out-crop of the Arrialoor group 

 is owing to the sedimentaiy accumulations having been more abundant here, and as in the 

 case of the Ootatoor Group deposited over a larger area, the later deposits being continually 

 swept to the outer edge of the area. 



