1836.] 



hetween Madras and the Neilgherries* 



21 



ed hereafter), having an intermixture of a little hornblende, are some- 

 what similar to those of Palliconda. 



On the surface of some of these porphyritic masses, the crystals of 

 felspar are more than an inch long, and project a little above the sur- 

 face of the rock, on account of the partial decay of the imbedding 

 paste. In the blocks of the porphyry which are near the sienite, there 

 are nests of hornblende, and then the porphyry in contact with it has a 

 reddish tint, probably from being penetrated by a little oxide of iron of 

 the hornblende ; of this porphyry the large blocks have a rounded form, 

 the small ones are prismatic, pyramidal, &c. 



Judging from the colour and appearance of the masses on the sum- 

 mit of the ridge, they are also porphyritic. 



Proceeding eastwardly, about two miles along the foot of the hill, we 

 meet two hillocks formed of sienite (No. 69), analogous to that of 

 Palliconda, and having, like it, some thin veins, or grains, of chlorite. 

 - In both these rocks I remarked many fissures, of which the direction 

 is invariably perpendicular, never parallel, to the horizon, so that they 

 assume prismatic, pyramidal, columnar, or spheroidal, forms. This 

 sienite decomposes into a red soil. 



Returning to the Bungalow, about half a mile from the foot of the 

 hill, I saw jutting out from the soil, three or four large oblong masses, 

 only a few feet above the ground, of a rock rather interesting, and 

 rarely met with in India (No. 70). 



It belongs to the endless varieties of the hornblende rocks, which, 

 Proteus-like, change appearance and composition in every locality in 

 this part of India. It is of a black colour — texture loose and friable- 

 traversed by veins of quartz and of felspar. It is composed of bottle- 

 green hornblende, intimately mixed with nearly the same quantity of 

 black mica, in a greater state of hardness than usual. I think it is the 

 same rock mentioned by the late Dr. Turnbull Christie, as found with 

 numerous other varieties of hornblende rock, at the stupendous falls of 

 Garsipa («). 



It is to be remarked that the portion of the rock, in contact with the 

 felspathic or quartzy veins loses its colour, becoming of a light green, 

 and powdery. Basaltic boulders are not rare in this plain. 



The three rocks, already described as found near Tripatoor, are 

 evidently unstratified; and, judging from the desultory examination of 

 a small portion of the hill, it would seem that the porphyry overlays 

 the sienite. It would be desirable that a thorough examination should 

 be made by some geologist, who could devote a few days to the geolo- 

 gical survey of this interesting mountain ridge. 



Gneiss appears to underlay these three rocks already described — 



(a) Edinburgh New Philosophical Journal, 1839, p. S. 



