Chap. III.] post-cuetacrous rocks. — (cuddalouf. sandstones). 87 



In the first small nullah which runs under the high road about 1 1 



miles east of "\'^ellumj and reaches the low ground 

 Pot holes in the grits. 



to the north of PuUayaputty, are several small but 



well marked and instructive examples of pot holes formed by the grind- 

 ing action of pebbles rapidly rotated by the eddies in the stream. 

 In several of the pot holes the pebbles were still lying, the force of the 

 stream not having been sufficient to wash them out of the steep-sided 

 holes they had been instrumental in scooping. "Westward of Vellum 

 nothing more is seen of the mottled grits, the whole surface being 

 thickly covered with red soil mixed with black laterite. 



At Verdachellum, in a quarry lying north of the Polygar^s Bunga- 

 low, a bed of true sandstone is exposed, which is sufficiently compact 

 and tenacious to be worked into slabs and troughs. It is of a pale color, 

 varying from yellow to a pinkish drab. The most frequent character 

 sxiperj&cially in this locality, is that of a tolerably compact brick-red sand, 

 with here and there marly and clayey patches of small extent, but 

 almost invariably also of red colour. Very few good sections are to be 

 found over the country occupied by these rocks, which has necessarily 

 greatly hindered us in obtaining a sufficient acquaintance with their 

 detailed structure. 



The thickness of this formation in proportion to its wide superficial 



spread is very small, and may, with tolerable safety, 

 Thickness of grits. 



be estimated at about 100 feet. Between Tanjore 



and Vellum, where it is very extensively exposed, it rises to a height of 



not much more than 60 feet above the general level of the alluvial 



flat which lies at the foot of it, and here it is most likely exposed, 



judging from the very gentle easterly slope, nearly down to its face. 



In the Cuddalore plateau, the thickness of the formation would appear 



to be rather greater, if the height of its northern scarp be a correct 



index ; in other words, if the whole thickness of beds there exposed 



belongs to the Cuddalore series, which would appear to be the case. 



( 259 ) 



