Chap. XL. ] 
VIZAGAPATAM : GARBHAM. 
1086 
time there has frequently developed in this soft rock a number of 
parallel bands of psilomelane, often of the shining compact lead-like va- 
riety, found also at Avagudem (of which an analysis is given on page 100). 
These bands follow the original dip of the rock. At other times the 
manganese has segregated to form irregidar nodules and concretions of 
hard psilomelane and soft black manganese oxide (often pyrolusite) ; and 
these have been most certainly formed by replacement of the original 
felspathic rock. Fig. 2, Plate 54, shows a very good example of this 
replacement seen in an excavation at the western end of the deposit. 
In this photograph the white is kaohnized felspar-rock containing a certain 
amount of quartz. The dark part is partly wad and partly manganese- 
ore and was evidently slowly advancing its boundaries at the expense of 
the white. In the latter are isolated black patches of pyrolusite, one of 
which is seen just to the left of the head of the hammer. Fig. 85 shows an 
enlarged view of one of these masses of pyrolusite in process of formation. 
The outer zone or border of the patch, about ^ to 1 inch thick, consists 
of soft black ore, whilst in the interior of the patch the felspar grains 
^ig- 85. - Quartz-felspar -rock undergoing replacement by manganese oxide. 
