Chap. III.] post-cretaceous rocks. — (cudd a lore sandstones). 41 



irregular pipe -like cavities of a yellowish -brown color, which is more 

 clayey in its constitution than that of the surface. 



Though of so uniform a structure over great part of its extent, the 



Conglomeratic larcritc. ^^^^'''^'' ^''^""'^ ^"^^^^>^ conglomeratic in many 

 places, especially in the outlying patches south 

 and south-east of Trichinopoly. Here, as on the Brigade-ground, and 

 a mile and a half south of the Race-course, it encloses sub-angular frao-- 

 ments of quartz and gneiss, of sizes varying from that of a small pebble 

 to pieces of 3 or 4 inches in diameter. At times the quantity of these 

 is so great that in the latter locality they almost make a pavement, 

 having, as it were, its seams or joints of laterite. A conglomerate of 

 quartz fragments is, however, the most ordinary form. Occasionally, the 

 laterite is a coarse deposit of small particles of quartz in a matrix of fer- 

 ruginous sandy clay. 



The laterite characterizes the country over which it is developed 

 Physical features of ^J ^^^ peculiarly flat or table-land siu-faces, which 

 a erite country. ^^^ sometimcs of great extent, as may be well seen 



on the Vellum table-land, or to a less extent south and south-west of 

 Thoongoody. Where the ground rises above the general level, this 

 deposit laps round it, leaving an island of the subjacent rock, or where 

 parallel streams have worn their way as they flowed down from the 

 higher grounds, we find the laterite denuded from all but the low ridges 

 between the streams, showing its scarped edges in contour lines along the 

 sides and round the heads of the valleys. Most interesting examples, 

 on a small scale, of this last feature, are observable to the south of the 

 Trichinopoly Race-course. 



The probable unconformity of the laterite to the mottled grits of 

 Unconformity of late- Tanjore and Vellum is assumed for the foUowing 

 rite and grits. rcason : that the laterite greatly overlaps the grits, 



stretching for many miles westward over the gneiss rocks in the direc- 

 tion of Trichinopoly. Of the existence of the overlap, there can be no 

 ¥ ( 'ZeS ) 



