THE HILLS OF SITABALDI, NAGPUR, &c. 125 



This rock is succeeded by an indurated clay or wacken, which at its 

 junction with the basalt, frequently partakes of its semi-columnar struc- 

 ture, the vertical fissures being prolonged into the wacken, and the sliis- 

 tose structure of the latter extending into the basalt : these changes may 

 be very favorably seen in the fosse, which surrounds the fortification. 



The indurated clay or wacken seems to form but a small proportion 

 of the hill, as it is not seen in the hollow between the greater and less 

 elevation, the semi-columnar basalt being there uncovered by any rock. 

 It also contains crystallized carbonate of lime and semi-opal, but calce- 

 dony coated with green earth, is the most common mineral found in it. 



This rock passes into the nodular wacken, which, on a casual inspec- 

 tion, appears to be a collection of stones rounded by attrition, and involv- 

 ed in a matrix of clay ; when examined more carefully it is evident that 

 this appearance is owing to a peculiar modification of the concretionary 

 structure, developed by decomposition. Nuclei, of various sizes, are enve- 

 loped 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 dif- 

 ferent lamellae, which surround their respective nuclei ; the centres from 

 which this pseudo crystallization has proceeded. The existence of the 

 vertical and M^aved 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 twenty-five per cent.) as may be perceived by the great 

 specific gravity of hand specimens. 



I 2 The 



