306 
A PECULIARITY IN THE STRUtTIURE OF GNEISS. 
near the liver are majestic tamarind trees, 
tending to add to the beauty of the scenery. 
On both sides the river the country is alluvial 
and sandy, and for two or three miles inter- 
spersed with hills and knolls: rounded 
blocks, like logging stones, are observed 
on their summits and sides. Dr. Benza 
discovered on a ridge, divided by a tor- 
rent from a high hill specimens of sienitic 
granite and a few plates of mica in addition 
to the other three minerals. Among them 
he found one resembling an analogous rock 
he found at Chinnapatam like red porphyry, 
but having nothing porphyritic in its struc- 
ture, being composed of red foliated felspar — 
fracture rather shining— honey-combed with 
numerous small cavities, filled with yellow 
substance, some being a micaceous, bril- 
liant, metallic powder, strongly magnetic. 
Our author advances to Baitmungalum 
where he feasted on grapes, peaches, and 
apples, of an exquisite flavour, the produce of 
Bangalore and Palamanar: the climate is re- 
presented as mild. Leaving behind the east- 
ern ghats, our author found rocks of gneiss, 
the contorted strata of which are seen al- 
most in every one of the blocks. A mile to 
the west near Golcondapatam blocks of 
granite are scattered over the plains. In the 
dry bed of a river Dr. Benza discovered a 
thick basaltic dyke which stretched across 
the whole breadth of the river, its outgoings 
being split into rhombs or parallelopi- 
peds, the dyke, appearing to burst through 
granite. The blocks of granite scattered 
over the plain resembled the erratic boulders 
found in the plains of Sweden, Russia, and 
Northern Germany, derived from the Scan- 
dinavian mountains. The granite exfoliates 
in concentric laminae of different thickness. 
In the sienitic granite nests of greenstone 
porphyry are imbedded, as is the case in 
almost all the localities in India where 
this rock is found. Our author proceeds 
west towards Shamarpilly, near the road 
to which is seen, a little oblong knoll, 
or rather undulation of ground on 
the tops of which rise many blocks of horn- 
blende rock CO ntaining very little fel- 
spar: structure semi-foliated, fracture glim- 
mering, quartz veins intersect it irregularly. 
The direction of the dyke-like bed of horn- 
, blende rock is north a nd south ; its decom- 
position imparts to the soil in its vici- 
nity a red ferruginous colour ; the oxide of 
iron appears, says our author, to enter largely 
in the composition of the hornblende, since, 
^ike other primitive green stones, it affects 
the magnetic needle. Among the rocks, be- 
fore reaching Bangalore, gneiss seems to pre- 
dominate ; it iscomposedof theusual mineral, 
forming regular strata conformable to each 
other, in some of which at onetime the mica, 
and at others the quartz, predominates. 
The quartz is white and . transparent, the 
felspar of a paler hue, and the mica black. 
Our author now reaches Bangalore, in the 
vicinity of which gneiss is seen every where, 
having veins of quartz or of foliated felspar, 
or of both together, traversing it. It is 
decomposed to a depth of seventy feet; the 
loam resulting from it is very abundant all 
about Bangalore. The clay found is excellent 
for tiles, bricks, &c. 
Dr. Benza notices here a peculiarity in 
the structure of gneiss ; namely, that of 
splitting, both naturally and artificially, 
into lamina, the direction of which is nearly 
perpendicular to that of the seams of the 
strata. 
“ In fact, in the laminae naturally detach- 
ed from the rock, we observed that the strata 
are seen either horizontal, or vertical on the 
surface of them ; therefore the laminae exfo- 
liate in a direction at angles with these strata. 
We see in all stratified rocks that they ge- 
nerally split in the direction of the strata ; 
so. that the surfaces of separation shew only 
the surfaces of the seam. But, in the laminae 
of this gneiss, the case is different ; on the 
surfaces of the laminae we see the strata, and 
their seams along the surface of the split, 
and therefore its direction is at an angle 
with that of the seams themselves.” 
The following is worthy of notice. 
“ It appears that the natives have availed 
themselves of the peculiarity this gneiss has, 
of splitting in a direction opposite to that 
of the strata, to obtain laminae of any thick- 
ness. The process to that effect is very 
simple and economical. On the convex sur- 
face of the gneiss they light a fire, the inten- 
sity of which is proportionate to the thick- 
ness of the slab to be obtained ; and, after 
having kept it up for such a length of time, 
as experience has taught them necessary for 
the required thickness, they extinguish it, 
and pour cold w'ater on the heated surface of 
the rock. 
This sudden refrigeration producing an in- 
stantaneous contraction of the heated portion 
of the rock, extending as deep as the heat 
had penetrated, it is detached at that depth 
