PART 4.] 
191 
Foote: Geology of North Areot District, 
Group I .—The MetamorpMe or Gneissic rods. 
It has already been pointed out that the rocks belonging to this group occupy 
by far the greatest part of the whole district. The predominant varieties are 
the massive obscurely bedded ones known among geologists as granite-gneiss and 
syenite-gneiss. The well-bedded flne-gTained schistose varieties are much less 
frequently met with than in other parts of the great gneissic region, as in the 
Trichinopoly, Salem, Madras, and Nellore districts, but they do appear locally 
among the coarse granitoid varieties, and occurrences of them will be described 
further on. 
As in all the gneissic regions, the rocks show abundant signs of having been 
much disturbed, contorted and uptilted since their original deposition, and 
finally forced into series of great synclinal and anticlinal foldings, the edges of 
which may in some cases be easily traced along great distances. 
A study of the direction of the edges or outcrops of the beds, technically 
called the “strike” of the bedding, shows that in the 
Prevalent strike. south-eastern part of the district the beds have a strike 
from south-south-west to north-noi’th-east, but as they are traced northward they 
are found to trend till they run north-south, which is the prevailing strike in the 
central parts of the district. If followed into the extreme northern part, they 
will be found to have trended to north-north-west. This change in the strike is 
part of a great curve formed by the metamorphic rocks in the latitude of Madras, 
and affecting the whole gneissic series eastward of the Mysore plateau. 
In describing the genei-al appearance of the central part of the district, Mr. 
Oldham in his notes remarks very aptly: “ Speaking generally, the whole area 
might be said to be one of quartzo-felspathic gneiss, commonly syenitoid or grani¬ 
toid.” “ When, however, I say qnartzo-felspathic, I do not mean to imply that these 
minerals only constitute the rock, but that they preponderate largely, and that 
the hornblende and mica which also enter into its composition play a very subordi¬ 
nate part, except in occasional bands.” These remarks apply equally well to the 
general features of the rugged flanks of the plateau edge as seen in the Sainegunta 
and Mugli Ghats and at Satghur, and to the whole western part of the district in 
fact. They apply also fully to the gneiss of the eastern side of the district north 
of the Palar, but less so to the rooks of the south-eastern talnqs. Here and 
around Vellore the well bedded varieties are rather more largely developed, and 
they are much less largely quartzo-felspathic in constitution. 
Commencing in the western part of the gneissic area, the eastern edge of the 
Mysore plateau consists mainly of coarse granitoid gneiss, which is well seen in the 
bills west of Ambur, at Satghur, and thence generally on the Sainegunta and 
Mugli Ghats leading to Palamaner, all around that place and northward of it, 
very conspicuously in the Gam Kondas and other hills stretching away to Avul- 
pilli Drug. The same description applies to the whole of the country between 
Vellore and Chittur, and to the very hilly tract lying betweem the Poiney river and 
the Madras railway. East of the railway too the granitoid gneiss extends under 
the younger Kadapa and Gondwana rock series. 
In the southern part of the gneissic area, forming the northern end of the 
B 
