ClIAr. III.] POST-CRETACEOUS ROCKS. (CUDDALORE SANDSTONES). 39 



brown friable surface of sandy clay to the true rock internally ; the 

 folia of quartz still remaining as needles or ridges in the decomposed 

 parts of the block. This is identically the same as the pseudo-lateritic 

 formation on the Nilg-iri and Shevaroy Mountains, which is there a 

 decomposed gneiss. 



The laterite proper consists essentially of an agglomeration of little 

 rounded particles cemented together by a ferruginous sandy clay, the 

 little nodules or concretions being more distinct in their form towards 

 the upper surface, where they become darker in color, gradually changing 

 from a yellowish-red to dark brown or black, eventually becoming quite 

 polished, and assuming a semi-metallic lustre. Generally, these consist 

 of ferruginous sandy clay, concretionary in structure, but in many cases 

 they are composed of iron-ore, showing a grey metallic surface when 

 broken ; grains of sands also are frequently included in the mass, which 

 sometimes assumes a true conglomeratic character. The matrix or 

 cementing material increases in quantity from the interior of the bed 

 to the sm-face, the latter being harder and less friable than the interior, 

 which is clayey. In its least compact form, this formation occurs as a 

 gravel-like accumulation of small rounded pellets of impure, clayey, 

 brown Haematite, for which the very suitable name of ^^ pisiform lateri- 

 tic graveF^ has been proposed by Dr. Oldham. Beds or large nests of 

 pale yellow and white clay occur with the laterite, as may be seen on the 

 eastern edge of the Thoongoody patch in the Trichinopoly district, 

 where a hole has been excavated for road material down to this clay, 

 which is quite soft, and when, as is often the case, covered over with 

 a thin coating of blown lateritic gravel, peculiarly deceptive in its 

 appearance of firmness. This case of soft clay would seem to indicate 

 an underlying variety or Lithomarge, but no section was found suffi- 

 ciently deep to show the relation between the clay and the underlying . 

 rock, which is gneiss. 



(261) 



