Chap. XL. ] 
VIZAGAPATAM : GARBHAM. 
1089 
Scale l"= 18 feet. 
A. — Lithomarglc clay with crystals of vein-quartz, patches of kaolinized quartz-felspar-rock and 
segregations of psilomelane. 
Fig. 90. — Section seen at the eastern end of the Garbham manganese-ore quarry. 
kaolinized, and often manganese-impregnated, quartz-felspar- or felspar- 
rock, as shown in Fig. 90. 
The mass of rocks forming the ore-body has been, especially at the 
east end of the deposit, injected by pegmatite 
Pegmatite and quartz ^ ^^^^ traverse it in all directions. This 
crystals. ' 
pegmatite is often a medium- to coarse-grained 
quartz-microcline-rock ; but it is much more frequently composed almost 
entirely of crystalline quartz taking the form of hexagonal prisms 
terminated by pyramid faces at one end, which usually points into the 
centre of the vein, the other end being implanted. These crystals range 
in size from very small dimensions up to a length, in one case, of 19 
inches with a diameter of eight. These veins were formed, in all 
probability, before the production by chemical action of the manga- 
nese-ores ; for the manganese-bearing solutions have filled in any 
cracks and spaces between the crystals with psilomelane and have often 
gone further and partly replaced the quartz so as to produce a breccia of 
residual quartz in a matrix of psilomelane. In some places this supposed 
siliceous pegmatite apparently takes the form of a siU, as in Fig. 90 above. 
It has been found continuously all the way from Hilltop to the extreme 
east end of the mine. 
There often occurs in the midst of the ore- bodies abundance of brown 
^^^^^ chert, probably formed simultaneously with the 
ore, and thus representing, as at Kodur, Perapi, 
