Chap. VIII.] arrialoor group — trichinopoly district. 137 



similar excavations as far North as Cootoomoor and Thoongapooram, little more can be made 

 out of them than that the succession of beds appears to continue much the same as I have 

 just described. The ridge running up to Cootoomoor is foi-med by the fossiUferous calcareous 

 bands which wc have traced up from the East of Arrialoor ; and that terminating at Thoon- 

 gapooram is formed by the coarse sands, some hard beds of which have probably enabled 

 them to resist denudation better than the soft sandy clays and friable shales which intervene 

 between them and the calcareous bands. It is probable also, that the Kothavassel capping 

 of Cuddalore sandstones extended along the ridge to Thoongapooram, almost till the close of the 

 final denudation, and these beds, cemented by iron and calcareous infiltrations into a tolerably 

 hard rock, have preserved the ridge in question, which is considerably above the average level 

 of the surrounding country. 

 With regard to the bottom beds, a few words more are necessary. Imperfect sections of 



these beds are exposed in the nullah to the East of Poothoor. 



Bottom beds. Olapaudy. ' 



(where, as in the Vevaghoor nullah, the bottom bed is seen 



resting with no apparent unconformity on the Trichinopoly beds) ; and also in some other 



small nullahs, which cut across the strike of the beds between Poothoor and Olapaudy. A t 



this latter place the bottom bed of the Arrialoor group is a coarse conglomerate, consisting of 



blocks of gneiss and Ootatoor limestone, (both of which are in situ a few yards from the 



spot,) imbedded in a soft sandy matrix. Some of the imbedded boulders were as much as 2 



feet in diameter, the majority not more than 6 inches to 1 foot, and most of the limestone 



blocks, even as imbedded, were covered with a polished ferruginous crust, similar to that 



characteristic of weathered laterite. The cause of this I could 

 Conglomerate. 



not clearly ascertain ; the amount of iron in the limestone being 



(as ascertained by analysis) very small ; so that the rock in situ shows no tendency to 



assume a similar appearance : yet the limestone blocks alone exhibited the peculiarity 



in question. The bed, which is well seen where it crosses the Cootoomoor road, is 3 or 



4 feet in thickness, and is followed by brown and grey sandy shales, which are exposed 



in some broken ground by the side of the road, and abound in fossils of nmnerous species. 



„ ., „, ^ Nautilus Bouchardianus is A-ery abimdant, the specimens beinff 



Fossils at Olapaudy. ' sr s> 



many of them of large size : several species of Ammonites, 



including A. sugata, Forbes, a Hamite, a Baculite, and species of the following genera, were 



also collected by Mr Chas. Oldham i — 



Rostellaria. Pecten. 



Fusus. Spondylus. 



Valuta. Inoceramus. 



Natica. Lima. 



Cerithium. Area. 



Turritella. Nucula. 



Turbo. Opis. 



Pleurotomaria. Lucina. 



Terebratula. Brissus. 



Rhynchonella . Nucleolites ? 



Crania. Cidaris. 



Ostrea. Marsupiles. 



Serpula. 



S 



