104 GEOLOGY OF TltlClllNOPOLY^ &C. [ChAP. IV. 2. 



a trap-dyke at the Eashurmullay, tlie metamorphic effect resulted pro- 

 bably from the dry heat emanating- from the continuation of the causes 

 which produced the mass of basaltic rock. 



The black porphyritic rock described as occurring so largely in a 

 highly weathered condition in the Pullj^utty valley may possibly have 

 been an eruptive mass of trap which caused the metamorphic action to 

 be set up. 



If this was the case, it would account to some extent for the smaller 

 number of magnesite veins found at that part of the altered tract of 

 ground, as the heat in the immediate neighbourhood would certainly have 

 vaporized any Avater within a considerable distance till its surface had 

 greatly cooled down, when the magnesite veins now seen traversing it 

 may have been formed by precipitation in fissures caused by contraction 

 in cooling. The temporary continuation of thermal springs would ac- 

 count perfectly for the state of complete disintegration observable on the 

 surface of this black rock. 



The formation of such a vast extent of serpentine-like rock, traversed by 

 thousands of magnesite veins, may be yet more easily comprehended, if we 

 suppose that a great part of the area was originally occupied by extensive 

 beds of steatite and other forms of magnesian rocks, the western exten- 

 sion of which remains unaltered to the present day at Carrupoor, as 

 proved by the opening of the pot-stone quarries at that village. 



Of all the rocks and minerals at the " Chalk Hills," the constituents 

 could have been, and probably Avere, derived from the metamorphosed 

 azoic rocks of the immediate locality, with the exception of the chromic 

 acid of the chromate of iron. This probably came from some distant 

 subterranean source. 



Of the geological period of these phenomena little can be said ; there 



is no apparent clue to rt, nor have we any means of 



Age of the magnesite. , • • .i - -i. ^.^ ■ c 



ascertammg the contemporaneity, or otherwise, ot 



the different series of phenomena ; there, is, however, a probability of the 

 ( 326 ) 



