300 ON THE GEOLOGICAL STRUCTURfi 
The indurated clay or wacke seems to form but a small 
proportion of the hill, as it is not seen in the hollow be- 
tween the greater and lesser elevation, the semi-columnar 
basalt being there uncovered by any rock. It also contains 
crystallised carbonate of lime and semi-opal, but calcedony 
coated with green earth is the most common mineral found 
in it. 
This rock passes into the nodular wacke, which, on a 
casual inspection, appears to be a collection of stones 
rounded by attrition, and involved in a matrix of clay ; 
when examined more carefully, it is evident that this ap- 
pearance is owing to a peculiar modification of the concre- 
tionary structure, developed by decomposition. Nuclei, of 
various sizes, are enveloped by concentric lamellae, which 
peel off, as decomposition destroys their cohesion. They 
are consequently seen in various states of decay, and of 
sizes varying from several feet in diameter to several inches. 
But their true nature is easily discovered by the mutual 
indentation of the different lamellae which surround their 
respective nuclei, the centres from which this pseudo-crys- 
tallisation 1ms proceeded. The existence of the vertical and 
waved fissures need scarcely be adduced as farther proof 
that they are not the product of alluvial detritus. 
It appears most probable that they owe their forms to 
molecular magnetic attraction, since they contain a very 
large proportion of oxide of iron (nearly 25 per cent.), as 
may be perceived by the great specific gravity of hand 
specimens. 
The nodular wacke or basalt is one of the most common 
forms of trap in the extensive districts composed of the 
rocks of this family south of the Nerbuddah. 
It occurs perpetually in the extensive and lofty range of 
mountains situate between the Poorna and Taptee rivers, 
and appears to form their principal mass. It is found 
