256 
geological features of the plain near madras. 
Art. III. — Notes on the Geology of thg 
country between Madras and the Neil- 
gherry Hills, via Bangalore and via 
Salem, by P. M. Bknza, Esq., M. 
T). of the ISiadras Establishment. 
On the Geological position and associa- 
tion of the Laterite, or Iron Clay, 
formation of India ; with a description 
of that rock as it is found at the Red 
Hills near Madras, by R. Cole, Esq , 
of the Madras Medical Establishment, 
Secretary to the Asiatic Department of 
the Madras Literary Society and 
Auxiliary Royal Asiatic Society . — 
Madras Journal, July, 1836. J. B. 
Pharoah, Madras. 
Dr. Benza opens his article by giving a 
concise statement of the geological features 
of the plain near Madras, with the view of 
communicating the names and nature of the 
rocks in that direction. Granite is the 
lowest rock in almost all the localities of the 
plain ; it is composed of quartz, felspar, 
and mica. The fact, that granite was 
always the lowest rock, was ascertained on 
boring for water and excavating for tanks 
and wells. This rock is found in many parts 
of the plain on the ’ surface of the soil in 
clustered masses ; near the mount and the 
race-course these eminences of granite are 
intermixed with pegmatite. In addition 
to other minerals at the foot of Palaveram 
the granite contains garnets; in some places 
it loses the mica and becomes pegmatite. At 
the western extremity of the mount the 
granite rocks are decomposed, forming 
white clay. Dr. Benza infers from the super- 
ficial position of the granite over the whole 
plain of Madras, that boring for water would 
prove unsuccessful. Porphyry, formed of well 
defined and separate crystals of felspar im- 
bedded in a compact paste of a similar mine- 
ral, occurs in the plain between Guindy and 
Trimatoor. No hornblende, andbut few plates 
of mica are discovered in the porphyry. 
Hornblende slate, occasionally passing into 
hornblende rock, overlays the fundamen- 
tal rock in the little eminences of the plain. 
Thrt stratification of this rock is well deve- 
loped — the contorted strata being composed || 
of coarse materials ; some are in a more com- jl 
minuted state, forming a fine grained stratum, i 
The rock is composed of an endless variety of 
proportions of minerals ; in some blocks strata ii; 
of hornblende is exclusively distinguished, [. 
in others felspar and quartz ; in some simple 1' 
quartz, which occasionally intersects the j,: 
strata at all angles and in different directions {i; 
in very thick veins. Hornblende rock, in j| 
huge masses on the summits and sides of the jfi 
hills, is foliated, shining, and nearly black ; 
contains little felspar ; is stratified in ap- 
pearance ; and is the primitive greenstone , 
found ail over India. Fracture splintery, !j; 
texture tough and compact. 
Conglomerate laterite extends over the j; 
whole eastern part of the plain , overlaying the f! 
granite in many places ; it is found in two j; 
states — viz. undisintegrated and detritus: itis ' 
found in its entire state, on the banks of the I;: 
Adyar, overlaying the pegmatite. The detri- I;; 
tus has two geological positions : one as loose ji 
rounded pebbles, scattered over the sur- It 
face of the plain ; the other as substratum to 'jl 
the soil. Between the lateritic detritus and [i 
the granite is found, in some places of the j| 
plain, a stratum of nodular kankar ; in some || 
spots it resembles friable, calcareous tufa. |i 
Trap is met with in considerable dimensions j 
in loose blocks, or in dykes between Pala- 
veram and Trimatoor, where they are nearly f 
level with the soil or forming small swellings |: 
on the ground. The boulders and the dykes |r 
are composed of basaltic hornblende : these j; 
dykes are frequent in India. As we ap- j; 
proach the sea the surface of a portion of the i 
plain is sandy, intermixed with minute grains Ij 
of disintegrated garnets. In the clayey |i 
stratum inland, marine organic exuviae have ij 
been found. Ij 
Such is an outline of Dr. Benza’s '{ 
sketch of the geology of the environs of , ' 
Madras, by which he has rendered great |i 
service to science, and supplied an im- || 
portant desideratum in Indian geology. ! 
The work is highly creditable to this au- i 
thor’s perseverance and zeal. Before we ac- 
company him on his journeys to the ji 
Neilgherries, we must examine Mr. Cole’s I 
