PART 4.] 
Fuote : Geology of North Arcot District. 
195 
I'lie felspathic porphyries occurring in North Arcot district are very interesting, 
as being the only examples of this species of rock known 
Felspathic porphyries. ■ i j. . 1 • i o i n ■ r 
in the south oi the peninsula, beveral very tine veins of 
this porphyry occur south of the Palikonda mountain (p. 192), and two of them 
may be traced for several miles in an east-to-west direction. In width they vary 
from 20 to 50 feet, and vary in color from smoky grey to pale drab, when freshly 
broken. Numerous prisms of flesh-colored or white felspar from Ith to ^ an 
inch long are included in the felspathic matrix. Small acicular crystals of 
dark green hornblende are less frequently included. The felspar crystals often 
stand out in gi-eat perfection on the weathered surfaces, and in some parts of the 
veins the included crystals form nearly half the mass. A little distance south 
of the western end of these two veins is a third in which the included small dark 
green acicular hornblende crystals are numerous, but the felspar crystals of rare 
occurrence in the gveyish-white felspathic matrix. The felspar crystals are white 
and show very faintly. 
On the east flank of the Palikonda mountain is a drab or pale buff-colored 
vein which appeal's to cross the mountain, for a precisely similar rock shows on the 
west flank. In this case the included crystals consist of felspar and quart?. 
Two other small veins of the same character appear on the we.st flank close to 
the north-west corner of the mountain, and a third of smoky grey coloi’, with 
reddish felspar crystals, runs nearly east-to-west for two or three hundred yards 
parallel with the high road to Bangaloi’e on the north side of the mountain. 
Another small vein of the felspathic porphyry occurs in the centre of the 
valley west of Kailasagiri peak (south-south-west of Vellore) ; it is of the same 
character as those just described. 
In point of age these porphyry veins seem to be newer than the trap dykes 
they cut in their course. Unfortunately this point could not be definitively 
settled owing to the presence of soil and debris at the points of intersection. 
Quartz veins are not very common. A few were noted by Mr. Oldham in the 
Wandiwash and Arcot taluqs; they are of good size (two 
Quartz veins. three miles in length), but offer no special features of 
interest; their course is either north-east-by-east to south-west-by-west, or north¬ 
west-by-north, south-east-by-south. 
The trappean rocks seen in North Arcot district all occur in the shape of 
dykes, often of great size and length, and forming im. 
portant features in the landscape. They occur commonly 
in all parts of the district, and in the central part occur frequently in very large 
numbers. 
The great majority of the dykes consist of coarse hornblendio trap, a form 
of greenstone, and there is in very many cases a direct proportion between the .size 
of the dyke and the coarseness of the rock composing it. Many of the dykes 
are markedly porphyritio in structure, including numerous ciystals of felspar in 
a hornblendio or hornblendo-felspathic matrix. If classified according to the 
directions of their courses, the dykes will bo found referable to two great systems, 
of which the one runs north to south, roughly speaking, and the other east-by¬ 
north to west-by-south. In the former, the course is less constant, and varies by 
Trap dykes. 
