1046 
MANGAMESE DEPOSITS OF INDIA : DESCRIPTIVE. [ PaRT IV : 
are all situated, as far as is known, in the coastal plains, but three 
occurrences of manganese-silicate-rocks (Nos. 21, 22, and 23, on 
Piate 56) have been located in the eastern fringe of the ghats, and a 
close search in these densely wooded hill tracts might lead to the 
discovery of economically valuable deposits of manganese. 
Geologically the Eastern Ghats, the eastern fringe of which is shown 
^ J on the map (Plate 06), are composed, in this 
part of their course, of alternating bands of 
gneissose granite, the charnockite series, and the khondalite series, with 
an average strike of N. N. E. The rocks in the plains to the east are 
largely obscured by alluvium given up to cultivation ; but wherever they 
appear at the surface either in isolated hills or mounds, or as obscure 
outcrops in the fields and stream-beds, they are found to be composed 
of rocks of the same three groups as in the Ghfits. These plains have 
not, however, been geologically mapped into these three divisions. The 
manganese-ore deposits mostly occur as low mounds and hillocks 
protruding from the alluvium, but in places the presence of manganese- 
ore beneath the surface is indicated by pebbles of ore at the surface. 
As has been earlier more fully explained (Chapter XIII), these 
deposits are regarded as having been formed by the chemical alteration 
of apatite- spandite -felspar-rock (kodurite), spandite-rock, pyroxene- 
spa ndite-rock, and other varieties of the kodurite series, with consequent 
concentration of manganese oxide into workable deposits of psilomelane, 
pyrolusite, and braunite. These manganese-silicate-rocks are supposed 
to be basic segregations from a magma that was considerably more 
acid (approaching a granite), and the whole mass of acid and basic 
rocks is held to have been intruded in the molten condition into the 
pre-existing rocks. The junction between manganese-intrusives and 
the older rocks is hnt rarely seen, but, judging from the relative prox- 
imity of rocks of the three series above mentioned, it seems as if the 
manganese rocks have always intruded themselves into the rocks of 
the khondalite series. The khondalite series of T. L. Walker consist 
chiefly of khondalite (sillimanite-garnet-graphite quartz-schists), and 
granular crystalline calcareous rocks composed of some or all of the 
following minerals : — diopside, woUastonite, scapolite, felspar, garnet, 
and oalcite ; and the manganese-intrusives seem to be more intimately 
associated with the latter rocks than with the typical khondalites. 
