PART 1 .] 
Geology of the neighbourhood of Madras. 
13 
The three principal kinds of rock above enumerated which make up the bulk of the 
lateritic series, namely, clayey conglomerates, gravels, and sands, are found constantly 
graduating into each other in such a manner that they can only be regarded as parts of 
one and the same deposit, however various the appearance of the different members. 
In the Madras area, and to the north of Madras in the Nellore district, the character 
of the lateritic formations is considerably different from that of the representatives of the 
same series further south in Trichinopoly and South Arcot districts, the difference consisting 
in the much greater frequency of conglomerates and in the presence of large quantities of 
quartzite pebbles enclosed in the conglomerates. This peculiarity depends on the proximity 
of the quartzite rocks of the Cuddapah series, which attain their southernmost point in the 
Naggery mountains, and even more perhaps on the extensive destruction of the much 
younger conglomerates of the jurassic series which were mainly composed of pebbles and 
boulders of such quartzite and were of great thickness. The laterites of South Arcot, 
Trichinopoly, and Tan)ore, oh the contrary, do not, so far as known, contain any quartzite 
whatsoever, hut only fragments of gneissic rocks, and these hut very rarely. The nearer the 
laterite of the Madras area approaches to the Alicoor and Sattavedu hills, which are entirely 
composed of the coarse jurassic conglomerate above spoken of, the larger do the quartzite 
and other pebbles it encloses become. In some of the laterite sections indeed near those hills 
the conglomerate is so coarse that few of the enclosed pebbles are less in size than a man’s 
head, and many very much larger; some, indeed, though perfectly smooth and well rounded, 
of such size as to deserve the name of boulders. 
In such conglomerates the matrix of ferruginous clay iron stone is almost masked by 
the included masses. 
Near the sea, however, as at the Red Hills, a few miles north-west of Madras, the 
included quartzite pebbles are small and not numerous, and the conglomeratic character not 
everywhere visible. Where such is the case, the peculiar characteristics of typical laterite, 
namely, its ferruginous character, its permeation by tubular and vesicular cavities filled with 
pale colored sandy clay, and lastly, its hardening and becoming coated with a glaze when 
exposed to atmospheric influences, are seen to perfection. 
The laterite gravels frequently contain pebbles of quartzite and gneiss mixed with the 
pisiform ferruginous pellets in varying proportion according to their proximity or distance 
from the older quartzite yielding rocks. The sands associated with the lateritic conglomerates 
and gravels show considerable variety in texture and color, the latter depending on the per¬ 
centage of iron. They not unfrequently contain a large quantity of clay, and are then apt 
to cake and harden, but without the excessive Assuring noticeable in purer clays. These 
sandy beds are frequently found overlying the highly ferruginous beds and form to a great 
extent the soil of the laterite areas. 
The Conjeveram gravels .—In the south-western part of the Madras area the high 
grounds north-north-east, and north-west of Conjeveram are covered by a peculiar quartzite 
gravel formation occupying the same relative position as the true laterite further to the east 
and north, but distinguishable from it by the absence of ferruginous matter, and conse¬ 
quently by its pale color. This change in mineral character (supposing the Conjeveram 
gravels to belong to the laterite period) takes place a little westward of a line drawn from 
Wallajabbad due north to the alluvium of the Cortelliar valley, but unfortunately no section 
occurs showing the two formations in juxtaposition. 
Both have so far proved unfossiliferous as far as true organic remains are concerned, but 
both appear to contain implements of human manufacture in the shape of axes and spear 
heads made of chipped quartzite pebbles and of the same types as those occurring in the 
gravels of Western Europe. I have given the name of the famous old town of Conjeveram 
to this non-ferruginous gravel deposit, from its occurring, so far as I know, only within the 
Conjeveram taluq. 
CUDDALORE SANDSTONES. 
Underlying the lateritic formations in the northern part of the Madras district is a 
gritty sandstone of white or drab color sometimes slightly mottled with rusty spots. This 
sandstone, which is well exposed only in the cliffs on the south side of the Cortelliar river, 
six miles north-west by north of the Red Hills, bears a strong resemblance lithologically 
