PREHISTORIC BOM HAY 139 



of whal before wore eminences on the main-land. Those would be 

 further worn down by the action of the weather and the waves, and 

 thus lend to be united into one, like Bombay in her present form, 

 by the deposit about their shores of the silt and sand formed by 

 their own detrition. 



But sneh elevation or depression, as that evidenced by the dip, of 

 tracts of country, is effected only by volcanic agency. It would seem, 

 then, that we owe our insular position to subterranean volcanic 

 forces. How energetic these have been in past times we have abun- 

 dant proof about us. 



If we proceed now to a more minute examination of the rock at 

 our feet, we find that it is heavy, hard, dark, crystalline, and on the 

 flat upper surface curiously marked with a net-work of whitish veins 

 into irregular hexagons. This points to a prismatic or columnar 

 structure of the rock, which, together with its other characteristics, 

 enables us to identify it with the class known to geologists as 

 " basaltic," which are formed bjr the solidification of ancient lava 

 flows. The columnar structure is not here so marked and general, nor 

 so regular, as at the well-known Giant's Causeway in Ireland, or the 

 Scutch Islands of Staff a and Iona, but unmistakeable groups of 

 hexagonal columns do occur. The most accessible just now is that 

 which overhangs the west side of the Peclder Road, a few yards to 

 the south of the over-bridge at the Parsi Towers of Silence. But 

 even where it is not fully developed in distinct columns, the pris- 

 matic structure can easily be traced in many places where the rock 

 has been exposed in vertical section by the operations of the quarry - 

 men. These also disclose the effect of decomposition through the 

 action of the weather, and the percolation of water. First the mass 

 splits into huge cubical blocks like those on the west side of the 

 Queen's Road, just below the ridge of Malabar Hill. These splits 

 take the line of the joints indicated by the network of white lines I 

 have mentioned, which are perhaps formed by the infiltration of 

 light coloured minerals in solution into the cracks caused by unequal 

 shrinkage of the mass of glowing lava while it contracts in cooling. 

 Where these blocks are allowed to lie undisturbed, in such a position 

 as to be exposed to the atmosphere but sheltered from any violent 

 action of the weather, thev are found to be surrounded with a red 



