288 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. f PaRT II : 
evidence makes it clear to me that both have been formed by im- 
pregnation and metasomatic replacement of Dharwar rocks at the 
surface. [This statement does not apply to all varieties of laterite.] 
I do not propose to give here any detailed accomit of the geology 
and origin of the Dharwar manganese-ores of these less metamorphosed 
types. 
These subjects are, however, touched upon in the description of each 
district, the fullest accounts being given for Jabalpur and the Sandur 
Hills. The lateritic and lateritoid ores are dealt with later under the 
heading of laterite (see pages 380 to 389). 
The More Complete Metamorphism of the I)hg,rw^rs. 
When the intensity of the metamorphism to which the Dharwar 
sediments were subjected was considerably more severe than described 
in the previous section, not only were the resultant structural 
changes more pronounced, but they were usually accompanied by a 
complete mineralogical re-arrangement or rather reconstruction of 
the rock. Sands were converted into vitreous quartzites in which 
any impurities crystallized out as definite minerals such as 
muscovite, hematite, garnet, or tourmaline, the formation of the 
last-named mineral possibly being due to the accession of various 
vapours to the mass of rock imdergoing metamorphism. Very impure 
quartzites, grits, and conglomerates, often became completely recrys- 
tallized with the total obliteration of their original characters 
and the formation, in at least some cases, of gneisses, this change 
depending, of course, on the presence in the original sediment of a 
sufficient quantity of felspathic or other alkali-bearing material to 
give rise to the fresh felspars of the gneiss formed by meta- 
morphism. Argillaceous sediments, such as clay, passed beyond the slate 
and phyllite stage and became entirely recrystallized as mica-schists, 
often crowded with various accessory minerals, such as rutUe, tourmaline, 
Primary man'^anese ottrelite. In the case of those deposits of 
^rea formed by the manganese-ore that were originally laid down in 
mfn^neTe^oxTdesedil ^he comparatively pure condition, the only effect of 
ments. the metamorphism can have been to compress the 
ore into the smallest possible volume and consequently compel the 
various constituents of the ore to arrange themselves as the mineral 
aggregate that occupies the least volume, that is, to bring about 
the formation of the minerals of the highest specific gravity. Any 
