144 
Records of the Geological Survey of India, 
[voL. xir. 
The alluvia. 
Granular 
gneiss. 
The latoritic area is divided by the alluvial valleys of the several rivers 
above enumerated into various patches, of which the most northerly are by far 
the largest in area. These patches will be found described and named further 
on. 
Of the alluvia there is very little to say. Only a strip of the southern part 
of the Cauvery delta was examined. The marine allu¬ 
vium forms but a very narrow belt in the Adrampatam* 
corner, and the river alluvia are of no great extent or im] 30 rtance. Owing 
to the great extent of wet cultivation carried on along the vai'ious rivers and 
under tanks constructed across their tributaries, the apparent area of the allu¬ 
vium has, in the course of many centuries, been largely increased by the forma¬ 
tion of artificial alluvial sj^roads, the boundaries between which and the true 
alluvia it is in very many, if not in most, cases impossible to determine with any 
accuracy. 
The several rock groups will be most conveniently studied by taking them in 
ascending order. 
II.— The gneissic or metamoephic rocks. 
The prevalent form of gneiss in this region is quartzo-felspathic micaceous 
granitoid or semi-granitoid gneiss, of pinkish or greyish- 
quartzose colour*. In texture it varies from a massive, coarse, 
highly granitoid rock to a schistose gneiss nearly akin to 
mica schist. A very marked variety which is of common occurrence is a coarse 
granular quartz rock very rudely bedded and showing numerous small indistinct 
cavities from which some mineral has been weathered out. In some cases these 
cavities are filled w'ith an earthy form of dark red or brown hfematite. The 
cavities He in the planes of lamination (coinciding with the bedding), and 
indeed but for the cavities the lamination would not be visible in most cases. 
The mass of the quartz is in places not unfrequently very translucent and 
vitreous in texture. 
Hornblendic varieties of gneiss are very much less common in this region, 
and talcose or chloritic schists were nowhere observed. Ferruginous schists 
are extremely rare; no example of hosmatite schist was met with, and only one 
example of magnetite schist. 
Finely banded granite gneiss of dense grain occurs here and there largely, 
as at Tirkornum, west of Pudukotai, and at Ammachattram on the Triohinopoly 
road. 
The line of hills already referred to (page 143) which cuts the Triehinopoly road 
south of Kolatur, and forms the Alurruttimallai" and Narthamallai, consists 
of banded slightly hornblendic granite gneiss of pale grey color weathering to 
pale dirty ilesh color, and showing characteristic bare rocky masses. Tors are 
not remarkable, or abundant, but there is much weathering along the lines of 
' Properly tlie name of tliis place should be called Adivira-riima-jiatiiain. 
^ The Alurrutti Malhii or “ Man-rolling hill” obtained its name from the practice adopted 
in former times of executing criminals hy rolling them over the great precipice on the south 
side of the hill. The hill is about 400 feet in height, and the upper part of the great south scarp 
overhangs slightly. 
