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inches in length, and a cubical piece of about four inches side, with a block weighing
300 lbs., were sent by him to the Madras Museum. Dr. Warth, of the Geological
Survey, considers them of great importance as indicating the probability of a large and
continuous yield. The quality of the quarried pieces is very little inferior to that of
the crystals. The specific gravity of the large crystals was 4*02 and of the rock
corundum 3 "So.



IMBEDDED MINERALS 17

Ores of silver have been said to occur in Belli Betta near Attikuppa.^
Ainslie states that Captain Arthur discovered this metal in small
quantities in Mysore, both in its native state in thin plates adhering to
some specimens of gold crystallized in minute cubes, and mineralized
with sulphur, iron and earthy matter, forming a kind of brittle
sulphuretted silver ore.

Gold has long been found in the alluvial soil bordering on the
Betarayan hills in Kolar District. The geognostic position of gold in
this and other localities appears to be in the i)rimary schists, viz., gneiss,
mica slate, clay slate, and hornblende schist, particularly near the line
of their contact with granite or basaltic dykes, where we generally find
the tendency to siliceous and metallic development unusually great.
The gold is almost invariably discovered either in thin veins or dissem-
inated in grains in the veins and beds of quartz, associated with iron
ore and sometimes platinum, and alloyed with small proportions of
silver and copper, or in the tracts of alluvial soil, beds of clay and
sands, with the washings of primary rocks. Mining operations were
carried on here by the natives from a remote period and abandoned.
But since 1875 gold mining has been revived on a large scale by
European enterprise, and what was virtually a desert waste has thus
been converted into a populous and thriving industrial centre. The
details of these operations will be found farther on under Industrial
Arts.

Plutonic Rocks.— Cm^/Vt' prevails throughout the great hypogene
tracts, sometimes rising abruptly from the surface of immense level
plains in precipitous peaked and dome-shaped masses ; sometimes in
low steppes ; sometimes in great heaps of amorphous masses ; at others
with sharp outlines, obscured and softened down by a mantle of the
hypogene schists which have accompanied its elevation. This latter
occurs most frecjuently in continuous mountain chains, such as the Ghats ;
but to view this rock in all the boldness of its true physical contour, we
must approach the detached ranges, clusters, and insulated masses that
break the monotony of the table-lands. Here we find but little
regularity in the direction of elevation. In many clusters the granite
appears to have burst through the crystalline schists in lines irregularly
radiating from a centre, or in rings resembling the denticulated periphery
of a crater.

The most remarkable of the insulated clusters and masses of granite
on the table-land of Mysore are those of Sivaganga, Savandroog,

' But Mr. Bruce Foolc, of tlic Geological Survey, reported in 1SS7 as follows ;

" I searched the hill most carefully and could not find the slightest trace of any ore of
silver."



1 8 GEOLOGY

Hutridroog, Nandidroog, Chandragutti, and Chitaldroog. The rock
of Nandidroog is almost one solid monolithic mass of granite, rising
1, 800 feet above the plain and upwards of 4,800 feet above the sea ;
that of Sivaganga is nearly as high. These masses have usually one or
more of their sides precipitous, or at such an angle as to be inaccessible
except at few points. Most of them, like that of Savandroog, are so
steep as to admit of Httle vegetation, and present surfaces of many
thousand square feet of perfectly naked rock, in which the veins and
mineralogical structure are beautifully laid bare to the eye of the
geologist.

It is not to be understood that granite is to be met with only in this
abrupt amorphous form. On the contrary, it is sometimes found in
immense undulating layers like lava, rising little above the general
level of the country, separated by fissures and joints, and running for
a considerable distance in a given direction like a regular chain of hills.
The horizontal fissures often impart a pseudo-stratified appearance, and
when crossed by others nearly vertical, give the whole the semblance
of some huge wall of cyclopean masonry. The cuboidal masses com-
posing these walls weather by a process of concentric exfoliation into
spheroids. This process occurs often on a grand scale, and the ex-
foliated portions compose segments of circles of many yards radii.
This decay of lofty granitic masses produces some of the most
picturesque features of an Indian landscape; its strange columnar piles,
trees, and logging stones, which far excel those of Dartmoor in grandeur
and in the fantastic forms they assume. Some of these piles are held
together in the most extraordinary positions, and the blocks composing
them are found connected by a felspathic siliceous and ferruginous
paste, the result of the decay of the upper masses, washed down and
deposited around the joints by the action of the rain. There they
stand ; some tottering on their base, leaning over and threatening every
instant to topple down upon the unwary traveller ; others erect, amid a
ruin of debris at their feet, — silent monuments of the process of the
surrounding decay. Sometimes the summits of the higher elevations
are composed of immense monolith peaked masses of granite, which
split vertically ; the separated portions are often known to descend
from their lofty position with the rapidity and thunder of an avalanche.
As the rocks waste from the summit, at their base will be usually
observed a tendency to a re-arrangement of the component particles
of the rock going on in the debris there accumulated. At Chitaldroog
may be seen, at the base of a granite clifi' which tops one of the hills,
a porphyritic-looking mass thus formed of a reddish clayey paste,
imbedding reddish crystals of felspar.



PLUTONIC ROCKS. 19

Almost every variety of this rock is found, but the prevaiUng granite
is composed of felspar, quartz, mica and hornblende. Quartz, felspar
and hornblende, the syenite of some mineralogists, is also common, and
runs into the ordinary granite. That beautiful variety called protogine,
in which talc, or chlorite, or steatite replaces the mica, is not very
common in India, but is met with in a few localities in the west of
Mysore. In all these cases chlorite and talc are the replacing minerals,
the former predominating. Pegmotite, granite composed of quartz and
felspar, is frequently met with ; but the variety called graphic granite is
rare. Schist granite never occurs as a mountain mass, but is found in
veins or patches imbedded in ordinary granite. The same may be said
of actinolitic granite, or granite in which actinolite replaces mica. The
latter usually is most frequent in hornblendic granite, and the actinolite
passes by insensible gradations into hornblende. The felspar of actino-
litic granite is usually flesh or salmon-coloured. Porphyritic granite, or
granite having large crystals of felspar imbedded in ordinary or small-
grained granite, is common. The rock of Savandroog affords a good
example of the prevailing variety. It is composed of a granite base of
felspar, quartz, mica and hornblende, imbedding long pale rose-coloured
crystals of felspar. Fine granite porphyries are less frequently met with :
a beautiful specimen occurs in a large vein or dyke which traverses the
gneiss in the bed of the Kaveri at Seringapatam, nearly opposite the
sallyport close to which Tipu was killed. It is composed of a basis of
compact reddish and salmon-coloured felspar and a little quartz,
imbedding lighter-coloured crystals of the same, with needle-shaped
crystals of green tourmaline.

The great prevalent mineralogical feature in the granite of Southern
India is its highly ferriferous nature. The mica and hornblende
is frequently replaced by magnetic iron ore in grains, veins, and
beds ; and sometimes by fine octohedral crystals of the same, with
polarity.

Most of the minerals and ores described as occurring in gneiss are
also found in granite.

The ordinary granite is traversed by veins of granites both finer and
larger grained : the former pass into eurite, a rock in which all the com-
ponent minerals of granite are mingled together in one almost homo-
geneous paste. The minerals composing the larger grained veins are
often in a state of segregation and crystallization. The mica, instead of
being scattered in minute scales throughout the substance of the rock
is sometimes collected in large plates nearly a foot in length (used by
natives for painting on) ; the quartz in large amorphous nodules, or
hexahedral pyramidal prisms of equal length ; and the felspar by itself

c 2



2 GEOLOGY

in reddish layers and beds. The veins and beds of felspar are usually
reddish, and penetrated by fissures, which give a prismatic structure :
these fissures are often lined with compact felspar, coloured by actino-
lite, or chlorite, or with drusy crystals of the former mineral, which is
also found in nests. Milky quartz is segregated into large beds forming
chains of hills, usually containing nests and seams of iron ore, rock
crystal, and crystals of amethystine quartz. Both oval and lenticular
nests of hornblende and mica occur in granite.

Granite is seen in veins penetrating the hypogene schists. Good
examples occur near Seringapatam. In many situations granite appears
to have broken through the earth's crust in a solid form ; as is evident
from the sometimes unaltered and shattered condition of the strata
immediately in contact.

Eiirite is found throughout the granite and hypogene tracts, but
more frequently among the latter rocks, with which it often has all the
appearance of being interstratified ; in the granite it occurs in dykes.
The eurite of Seringapatam may be regarded as a type of the petrosilex
eurites. It sometimes passes into eurite porphyry, imbedding distinct
crystals of laminar felspar. Diallage, euphotide or gabbro, occurs at
Banavar, about eight miles westerly from Bangalore, associated with
gneiss and mica schist. It there presents itself in low elevations, con-
sisting of angular rough masses of the diallage rock, half-buried in a
detritus the result of its own disintegration. The masses have not the
slightest appearance of stratification ; but are divided by fissures, like
granite, into cuboidal blocks. The rock is composed chiefly of diallage
and felspar ; the colours of the former varying from light and dark grey
to greyish green and bright green. The felspar is white and greyish
white ; sometimes in distinct crystals, but generally confusedly
aggregated. The general colour of the rock is light grey and
greenish grey. The diallage at Banavar has more the appearance of
a dyke or vein in the hypogene strata than of an interstratified bed ;
but no natural section of the junction line of the two rocks presents
itself

Serpentine. — Near Turuvekere a dark crystalline rock occurs, com-
posed of a dark grey or black talcose paste, imbedding numerous small
black crystals of a mineral containing a large proportion of iron, being
strongly attracted by the magnet. It bears a beautiful polish ; the
surface exhibiting, on close inspection, in the dark shining paste, still
darker spots occasioned by the magnetic crystals. It was quarried by
the sovereigns of Mysore for architectural purposes, and forms the
material of the beautiful pillars which support the mausoleum of Haidar
at Seringapatam. This rock has been mistaken for basaltic greenstone,.



VOLCANIC ROCKS 21

but it may be a bed of massive ferriferous potstone — here common in
the talc schist — elevated, indurated, and altered by one of the basaltic
dykes that traverse the rocks in the vicinity. Geologically viewed it
has all the characters of a serpentine ; and mineralogically it resembles
the ferriferous serpentine or ophiolite of Brongniart, which consists
of a magnesian paste imbedding disseminated grains of oxidulated
iron.

Yolcanic Rocks. — Basaltic greenstone is universally distributed. It
prevails in hypogene areas, diminishes in those occupied by the diamond-
sandstone and limestone, and totally disappears in districts covered by
laterite and deposits of a more recent epoch. It is most developed in
the stretch of table-land between Bangalore and Bellary. It never
occurs in continuous overlying sheets like the newer trap, but pene-
trates in dykes the rocks just described, up to the age of the laterite.
These dykes often terminate on reaching the surface of the rock, or
before reaching it ; while others project from the surface in long black
ridges, which, originally like a wall, have since tumbled into both
globular and angular fragments by disintegration. Most of the blocks
usually remain piled up on the crests of the elevations, while others
have lodged on their sides or rolled down to their bases. Many of these
blocks have a peculiar metallic or phonolithic sound when struck ; the
well-known " ringing stones " west of Bellary afford a good example.
These black bare ridges of loose stones, standing out in relief against
the light-coloured granite or gneiss rocks, add another striking feature
to the landscape of the plutonic and hypogene tracts. They often
cross the country in a thick network, particularly between Nandidroog
and Bagepalli.

In many cases the protrusion of the basaltic greenstone above the
general surface of the imbedding rock appears to have been occasioned
by the weathering of the latter from its sides. The greenstone thus left
unsupported and exposed to atmospheric action soon breaks up by the
process of Assuring and concentric exfoliation. In a few instances it
appears to have been forced in a semi-solid state beyond the lips of the
rent in the rock without overlapping the rock, but none of these project-
ing dykes have remained in that solid continuous wall-like state in which
we see the prominent dykes of Somma or the Val del Bove. Their
height above the general level of the country rarely exceeds eighty feet.
The direction of the main dykes appears generally to coincide with that
of the elevation of the mountains ; but if we trace any dyke, the general
direction of which in a course of many miles may be north and south,
we shall find it to zig-zag and curve in various directions at different
parts of its course. Fragments of granite and gneiss, both angular and



2 2 GEOLOGY

of a lenticular form, arc sometimes entangled and imbedded in the
basalt ; and have been mistaken for veins or nests of these rocks. It is
evident that, in many instances, the granite and hypogene rocks were
solidified prior to the great eruptions of basalt that burst up from below
into their seams and fissures, and that the molten fluid imbedded all
loose fragments of rock, &:c., lying in them. It is probable that many
of the fissures themselves were caused, or enlarged, as seen in modern
volcanoes, by the expansion of the molten basalt and its gases from
below, while struggling for a vent.

The lithologic structure of this rock is as protean as that of granite.
In the centre of large dykes we usually find it crystalline and por-
phyritic ; and nearer the edges, less crystalline and more compact ; in
fact, every gradation of amj)hibolitic and augitic rocks, from basalt to
melaphyre, in the distance of a very few paces. Near the sides, in the
compact varieties, may be seen needle-shaped crystals of augite, glanc-
ing in confused arrangement here and there in the close texture of the
basalt ; while a little nearer to the centre the augite almost disappears,
and is replaced by fine large crystals of hornblende, and sometimes a
few scattered scales of mica. Near the line of contact with gneiss, the
basalt often loses its dark colour, and becomes of a faint green, like
some varieties of eurite or serpentine, imbedding iron pyrites. This
faint green eurite is also seen as a thin vitreous and vesicular enduit on
its surface, like the scoriaceous lava found on the surface of the dykes
of Etna. The cavities sometimes contain a yellowish-brown powder,
which becomes magnetic before the blow-pipe ; or small crystals of
epidote : in one specimen was found prehnite. The surface of the com-
pact basalt in the dykes is often scored by small fissures, which, as in
the Vesuvian dykes, divide the rock into horizontal prisms and run at
right angles to the cooling surfaces. All the darker varieties of basaltic
greenstone melt into a black or dark-green coloured glass or enamel ;
and affect the magnetic needle. They are composed of felspar, horn-
blende and augite, in varying proportions, and occasionally hyper-
sthene.

The minerals most common to these are, iron pyrites, garnets, epidote,
and actinolite. These minerals distinguish them from the newer trap,
which abounds in zeolites, calcedonies and olivine.

The greenstone occasionally assumes the prismatic columnar forms of
the newer basalts, or rather approaches to this structure ; thin layers of
carbonate of lime often intervene between the joints, and between the
concentric layers of the globular greenstone. In many instances the
basalt has a fissile structure, which, when intersected by joints, form
prisms well adapted for building purposes. In some cases, under the



AQUEOUS MOCKS 23

hammer it breaks into rhomboidal fragments, the joint planes of which
are marked superficially with dark brown or blue dendritic appearances
on a pale yellow or brown ground.

Rocks altered by Dykes. — Granite and gneiss in contact with a dyke
usually become compact, or tough, or friable ; the felspar crystals
lose their brightness and a portion of the water of crystallization,
become opaque and of porcelain hue; the mica is hardened and
loses its easily fissile lamellar character. In gneiss it may be seen
replaced by minute crystals of tourmaline,, epidote and garnet, as
near Chanraypatna. Limestone is converted into chert, or becomes
siliceous ; sandstone into quartz ; and clay slate into basanite and
jasper.

In districts most intersected by dykes a general tendency to crystal-
line and metallic development will be remarked, as well as an increase
in the deposition of saline and calcareous matter, apparent in extensive
layers of kunker, and efflorescences of the carbonate, muriate, and sul-
phate of soda. The fissures through which the springs charged with
these minerals rise, were originally caused, perhaps, by the same dis-
ruptive forces that opened vents through the earth's crust to the molten
basalt : and it is not improbable that these minerals and sulphates have
their origin in causes connected with these ancient subterranean
volcanic phenomena. Frequently no alteration is to be traced in the
rocks in contact with dykes ; a circumstance readily accounted for when
we reflect that the temperature of the injected rock is liable to great
variation. In certain localities, indeed, the basalt appears to have been
reciprocally acted upon by the rock it has traversed.

Aqueous Rocks. — Sandstone and Limestone. — Resting immediately
on the hypogene and plutonic rocks are found beds of limestone, sand-
stone, conglomerate, argillaceous, arenaceous, and siliceous schists.
Next to the hypogene schists, and the associated plutonic rocks, these
limestone and sandstone beds occupy perhaps the greater portion of
the area north of a line drawn through Sira to the west. They are
most frequently observed exposed in the vicinity of the great drainage
lines of the country and occur in irregularly-shaped patches, separated
usually by broad and apparently denuded zones of the subjacent
hypogene and plutonic rocks.

The tracts occupied by the limestone and sandstone beds present a
diversified aspect, sometimes flat and monotonous, and at others, near
lines of [)lutonic disturbance, bare, rugged and picturesque. The lime-
stone in some situations has evidently been denuded of the usually
superjacent sandstone, dislocated, and elevated several hundreds of
feet above the general level of the surrounding country in regular



24 GEOLOGY

ranges, and often in highly-inclined strata. Caps of sandstone, though
in such cases often wanting, are sometimes seen still covering the
limestone peaks. The outline of these limestone ranges usually
presents long, fiattish-topped ridges, whose sides and summits are not
unfrequently covered with detached angular blocks of the rocks, with a
grey, weathered, and scabrous exterior, resembling that of the mountain
limestones of Europe.

The sandstone, where undisturbed by plutonic intrusion, occurs in
low, flat, wall-like ranges, rising at an almost similar level, rarely exceed-
ing, 500 feet from the surface of the surrounding country, supporting
tabte-lands of some extent and evidently once continuous. It is often
intersected by deep fissures, extending from the summit of the rocks
down to the base. When disturbed by plutonic force, the sandstone
exhibits a striking contrast in its outline to the tame horizontal aspect
it assumes at a distance from the axes of disturbance. It rises in bold
relief against the sky in lofty rugged cross or hogbacked and crested
hills, with precipitous mural ridges, which, rarely running at the same
level for any distance, are interrupted by portions of the same ridge,
thrown up at various angles with the horizon in steep and often
inaccessible cliffs. When it crests the hypogene rocks, the lower
part of the elevation is often composed of the latter to the height of
about 200 to 400 feet, the slope of which has usually an inclination
of from 15° to 20", while that of the cap of sandstone presents a
steep or precipitous declivity varying from 45° to 90°, giving a decided
character to the aspect and configuration of the mountains and ranges
thus formed.

The hills of arenaceous schists are to be recognized from the more
massive sandstones by their undulating, round-backed summits, and
their buttressed and dimpled flanks ; while those of the softer slates
and shales affect the mammiform outline.

Both limestone and sandstone beds, there is little doubt, were
formerly of greater extent than now, and owe much of their present
discontinuity and scattered positions to the agency of plutonic
disturbance and subsequent denudation. The tracts of country
intervening between their areas are usually occupied by granitic and
hypogene rocks.

Laterite occupies a large portion of the superficies of Southern
India. It is found capping the loftiest summits of the Eastern and
Western Ghats and of some of the isolated peaks on the intervening
table-lands. Beds of small extent occur near Bangalore and Banavasi.
That at Bangalore extends northerly towards the vicinity of Nandi-
droog. Hills of laterite are usually distinguished by their long, low,



LATERITE 25

flat-topped character, assimilating those of the trap and horizontal
sandstone formations. The lands they support are, however, not so
much furrowed as those of the sandstone by water channels, a circum-
stance ascribable to the drainage passing rapidly off through the pores
of the rock. When capping detached rocks, the laterite usually imparts
to the whole mass a dome-shaped or mammiform outline, or that of a
truncated cone.

On the surface of table-lands it is spread out in sheets, varying from
a few inches to about 250 feet in thickness, terminating on one or two
sides in mural escarpments. Immense detached blocks, generally of a
cuboidal shape, are often seen occurring on the flanks of the Western
Ghats, and on the southern slopes of the Sondur hills, often separated
and dislodged. The valleys intervening between ranges of laterite
hills are generally winding, like those formed by the course of a
stream, and flat-bottomed, particularly in districts where it overlies the
newer trap.

The laterite varies mucli in structure and composition ; l)ut generally
speaking it presents a reddish-brown or brick-coloured tubular and
-cellular clay, more or less indurated ; passing on the one hand into a
hard compact jaspideous rock, and on the other into loosely aggregated
grits or sandstones, and into red sectile clays, red and yellow ochre,
and white porcelain earth, plum-blue, red, purplish and variegated
lithomarges. Sometimes it presents the character of a conglomerate,
containing fragments of quartz, the plutonic, hypogene and sandstone
rocks and nodules of iron ore derived from them, all imbedded in a
ferruginous clay. The cavities are both vesicular, tubular and sinuous ;



Online LibraryB. Lewis (Benjamin Lewis) RiceMysore: a gazetteer compiled for government (Volume 1) → online text (page 4 of 98)