658 Notes, chiefly Geological, [No. 165. 



about fifty miles N. of this. It is very evident, from their rolled aspect, 

 that these hard quartz pebbles have travelled, and been subjected to the 

 action of water in motion ; but whether they have been washed direct 

 from the parent rock to the place we now see them in, or whether 

 they were once imbedded in deposits of laterite on, or near the spot, 

 and which have since been swept off, is uncertain. A little farther to 

 the westward of the bungalow, the surface of the plain is strewed with 

 the harder debris of a bed of laterite, a circumstance in favour of the 

 latter hypothesis, and among which are rolled fragments of a chocolate 

 sandstone, exactly resembling those found by my friend, Cole, in the 

 laterite of the Red hills. Rounded pebbles of white and red ferruginous 

 quartz are also scattered on the surface, and beds of a fine light- coloured 

 sand, like that of the Egyptian desert, and evidently not the result of 

 the disintegration of rocks in situ. In short, there is every appearance 

 of this part of the Carnatic having emerged at no distant geological 

 period from beneath the surface of the water. 



From the little worn aspect of the fragments of the granitic rocks, and 

 the softer shale6, it is evident that these rocks are at no great distance 

 hence in situ : accordingly I continued my search in the plain to the west- 

 ward, and at length succeeded in finding the white shale in situ in the 

 bed of a small stream which feeds the tank, and on its banks a light 

 grey sandstone outcropping in the bed of a small pool ; both rocks in 

 horizontal strata, the sandstone overlying the shale. The sandstone is 

 rather coarse or granular in structure, being composed of angular 

 grains of greyish quartz held together by a white felspathic paste. In 

 some excavations a little to the east of the bungalow, it passes both 

 into a conglomerate imbedding small rounded pebbles of white quartz, 

 and into a ferruginous sandstone resembling that imbedding silicified 

 wood near Pondicherry. This sandstone, like the laterite with which 

 it is associated, has evidently been broken through, and stripped off 

 in many places by aqueous denudation, its strata being by no means 

 thick or continuous. 



It is found in the plain between Madras and Naggery in a more con- 

 solidated and compact form, and has been judiciously employed on 

 account of its containing but little or no iron, by Lieut. Ludlow, in the 

 construction of stands for the instruments in the Magnetic Observatory 



