2 



Geology of the Country 



[July 



such as to the right of the road from Madras to the Mount, just after 

 passing Marmalong-bridge — also a mile farther up, close to the right 

 bank of the river— near the little Mount— to the west of, and few yards 

 from, the Race-Course — in which places the clusters of granite are in- 

 termixed with those of pegmatite (No. 2)— at the foot of Palaveram 

 Hill, in one of which the granite contains garnets, in addition to the 

 other minerals (No. 3) — and probably in many other places, which 

 have not come under my observation. 



In more than one locality, this granite loses the mica and becomes 

 pegmatite, one of the granitic rocks most apt to decompose, forming 

 white clay, or kaolin (No. 4) ; as may be seen at the western ex- 

 tremity of the Mount — along the right bank of the Adyar river, 

 below Marmalong-bridge — between the native village, at the foot of the 

 Mount, and the Race-Course ; and in many other places. 



Considering this superficial position of granite over the whole plain 

 of Madras, it would seem more than probable that boring for water 

 cannot be attended with success in any part of it. 



Porphyritic boulders are not a rare occurrence in this plain, of 

 which I have seen some between Guindy and Trimatoor, and at the 

 foot of Palaveram. This porphyry is formed of well defined and sepa- 

 rate crystals of white felspar, imbedded in a paste of the same mineral 

 in the compact state (No. 5). Not a tract of hornblende, and very few 

 plates of mica, are found in this porphyry. 



In the little eminences of this plain, hornblende slate, occasion- 

 ally passing into hornblende rock, overlays the fundamental rock ; 

 such is the case at the Mount, at Palaveram, etc. (No. 6). 



The stratification of this rock is clearly seen in every place where 

 it exists ; many of the contorted strata being composed of coarse mate- 

 rials ; others although having the same minerals, are in a more com- 

 minuted state, forming a finer grained stratum. In this rock the variety 

 of proportions of the minerals composing it is endless ; in some blocks 

 we see strata of hornblende only; in others, of felspar and quartz, 

 and, in others, of simple quartz ; which last mineral occasionally in ter- 

 sects the strata at all angles, and in different directions, in very thick 

 veins. 



The huge masses of hornblende rock on the summits, or on the sides, 

 of these hills, contain very little felspar, and having the appearance of 

 being unstratified, the hornblende being foliated, shining and nearly 

 black; in short it is the primitive greenstone, which is found all over 

 India (No. 7). Its fracture is splintery, and the texture, like that of all 

 green-stones, very tough and compact. 



We must not omit mentioning in this place a rock which extends 

 nearly over the whole plain (at least the eastern part of it), overlaying 

 in many places the granite. I mean the conglomerate laterite, which 



