300 ON THE TKANSMUTATION OF EOCKS, &c., 



It is clear that the prismatic structure induced on the sand- 

 stones must have been effected by the trap in a very different 

 condition of these beds. Now, if we examine the prismatic 

 sandstone, we shall find that it is sometimes very absorbent, and 

 does not justify the opinion once formed of its durability. At 

 Meriberi, and at Five Dock it has been extensively used for the 

 roads. Being consulted about it some years ago by a surveyor, 

 I undertook an examination of it for his guidance ; and the 

 result was, that it was far from answering the expectation 

 formed of it. Some of it had a higher Specific gravity than 

 portions of unaltered sandstone, but of some the density was 

 lower. In 1864, Professor Smith obtained S.GL of some 

 unaltered rock, 2'41 and 2 '44, and of the altered rock 2*47. 

 In 1859, I obtained S.Gr. of other portions of white columns 

 2*37. The compact hard sandstone of Blackwattle Swamp 

 gave me 2'45 ; but at Pyrmont the Specific gravity of the 

 harder rock came out 3*01, and the softer 2'31. In general the 

 Specific gravity of transmuted sandstone is higher than the 

 nominal value. Thus, an altered red sandstone near Belfast 

 underlying a dolerite has a polyhedral structure, and its Specific 

 gravity nearly 7 per cent. (6 '43 per cent.) higher than the 

 unaltered. At Comber, in the county of Down, the Specific 

 gravity increased from 2'522 to 2'545. 



On the contrary, a white prismatic sandstone at Wildenstein, 

 very like the Meriberi prisms, having a light greyish blue tint, 

 was found by M. Delesse, to whom I am indebted for this example, 

 to have a lower Specific gravity than the unaltered rock which is 

 red, though the difference is not very great. The explanation is, 

 that the quartz was slightly vitrified, for the density of the sand- 

 stone fused artificially came out 2'081 instead of 2'342. It would 

 lead me away from my present purpose to discuss the question of 

 vitrification. 



Eespecting the transmuted rocks at Meriberi, near Bondi, 

 their white colour arises partly from the silicate of alumina with 

 which the grains are cemented. There is no vitrification of the 

 mass, only the prisms are generally clean, the joints cutting 

 through the larger pebbles of quartz in the coarser varieties. It 

 is such a change as might be anticipated from the action of heat 

 administered in water. 



