1836.] 



between Madras and the Neilgherries. 



3 



is observed in two different conditions, that of undisintegrated and 

 that of a detritus. 



The conglomerate laterite is seen, in its entire state, on the banks of 

 the Adyar (where the blocks for the Break-water are quarried), overlay- 

 ing the pegmatite — close to the Race-Course, going to the native village 

 under the Mount, on a granitic block; and perhaps in other places 

 (No. 8). 



The detritus from this rock has two geological positions ; the 

 one as loose rounded pebbles, scattered all over the surface of the plain; 

 the other as a substratum to the soil. This last has sometimes many- 

 feet thickness, imbedding occasionally undecomposed pieces of the com- 

 pact conglomerate laterite, which proves that the detritus is derived 

 from the present conglomerate (No. 9). 



In more than one locality of this plain a stratum of nodular kankar is 

 found between the lateritic detritus and the granite (No. 10). In some 

 of these places (Guindy-garden near the Tank, south of the House) it 

 is like earthy, friable, calcareous tufa, having pieces of granite imbed- 

 ded in it (No. 11). 



Trap is not unfrequently met with, either in loose blocks, or in dykes 

 apparently of considerable dimensions. These last are to be seen be- 

 tween Palaveram and Trimatoor, where they are nearly level with the 

 soil, or forming small swellings on the ground. Both the boulders and 

 the dykes are composed chiefly of basaltic hornblende (No. 12). These 

 dykes, as I have mentioned in another publication, are of a very fre- 

 quent occurrence in India (a). 



The surface of a portion of this plain, particularly near the sea, is 

 sandy in some places, having minute grains of disintegrated garnets, 

 which are derived from that mineral contained in the granite, and in 

 the hornblende slate of the Hills about (No. 13). 



Before finishing the geology of the plain of Madras, I must mention 

 that in the clayey stratum, which, in some places inland, underlays the 

 sand, marine organic exuviee have been found, according to information 

 I have received. 



The investigation of the existence of these fossils is of the highest 

 possible geological interest, and it would be doing the greatest service 

 to science, if zealous individuals, resident in the Carnatic and along the 

 Coromandel Coast, would collect specimens and facts on this subject, 

 which is one of the desiderata in Indian geology. 



Hitherto we have but few observations on the subject. Dr. Voysey 

 was, I think, one of the first who mentioned the existence of marine 

 and fresh water shells, in a fossil state, in the south of India (b). Colonel 

 Cullen of the Madras Presidency deposited, as far back as 1822, in the 



<«) Journal of the Asiatic Society of Bengal, August 1835, page 432, 

 (b) Journal of the Asiatic Society of Bengal, 1833, 



