32 CRETACEOUS ROCKS OF S. INDIA. [PaRT II. § 1. 



Of the elevation of the Thutchuncoorchy boss as far North as the 



-n ^ p , , c confines of the Cretaceous rocks, we can, however, 

 JJate of ui)hcaval of > j j 



^'^'^s^- ascertain the date with great exactness, and prove 



ujscqucn o o a oor, ^^^^^ ^^ occurred immediately subsequently to^the 

 formation of the Ootatoor group. During the formation of the newer 

 groups, or at least of so much of them as now remain after extensive 

 prior to Trichinopoly denudation, the Thutchuncoorchy tract formed 

 ^°"^^' a headland in the Cretaceous sea ; and the con- 



glomerates and sand banks, which abound in the lower beds of the 

 Trichinopoly and Arrialoor groups, are formed in great part of its 

 abraded constituents. In the lower beds, also, of the Cuddalore sand- 

 stones in the vicinity of Arrialoor, and many miles to the Northward, 

 rounded pebbles of quartz and orthoclase felspar are tolerably 

 abundant, and as no granite or any other rocks that could yield such 

 materials occur anywhere in the surrounding country or for many 

 miles to the Northward, there can be little doubt that these too were 

 derived from the abrasion of the Thutchuncoorchy ridge. 



The granite is for the most part a binary compound of quartz and 

 Mineral character of ^Ispar, sometimes compact and crystalline, but 

 ^^'^"'*^* more frequently with the felspar (a pale yellow 



Orthoclase) in large crystals or cleavable masses, more or less penetrated 

 v/ith quartz. In some of the crystals the disposition of the quartz con- 

 forms to the cleavage planes of the felspar, forming the so-called graphic 

 granite, but I have never observed any large masses of this peculiar rock. 

 The relative proportions of the constituent minerals are also subject to 

 much variation, and broad veins of almost pure quartz are by no means 

 unfrequent. Mica occurs but rarely, but when present generally assumes 

 the form of hexagonal plates not exceeding an inch or two in diameter* ; 



* The plates of mica upon which the native artists of Tricliinopoly paint figures of birds, 

 native costumes, local views, &c., are not dcri^■e(l from any local source, but are imported 

 from Bcii2;al, and arc from the mica mines of Beliar. 



