296 ON THE TRANSMUTATION OF ROCKS, &c., 



main entrance of the Custom House. One of the most ordinary 

 forms is a nodular, or concretionary arrangement, and these 

 concretions have sometimes a diameter of forty feet. 



Now, I class these changes and appearances among the 

 phenomena of transmutation. 



But, when prismatic action occurs colours on the external 

 surfaces of the stone are generally absent. The coloured portions 

 occur in the interior in patches or blotches ; and in the case of 

 the Five Dock quarry the columnar portions have a centre, 

 generally of rectilineal sides, in which the ferruginous matter 

 has been attracted, as it were, to that centre, or left when 

 the rock became jointed and gases passed upwards, deoxidising 

 the iron. The surfaces of these prisms are, however, speckled 

 with portions of a white and, where wet, pasty substance which 

 has evidently resulted from an external agent, and which puts on 

 the character of a transmutation. It is probable, therefore, that 

 the internal assemblage of ferruginous matters is occasioned by a 

 transmutation which has removed the colouring matter from the 

 surrounding parts. And yet we must bear in mind that, as the 

 silica is pure and generally transparent, and the silicate of 

 alumina, which varies sometimes in excess of one or other of the 

 ingredients, is also white, it is not always to transmuting agency 

 we must refer the whiter portions of sandstone in the 

 Hawkesbury series. 



In* various parts of the country I have noticed white bands 

 for some distance on each side of a crack in coloured sandstone, 

 and often there, where wide enough, the fissure is filled in with 

 silicate of alumina which thus forms apparently independent 

 dykes ; such as are near the east end of the wall of the Victoria 

 Barracks. 



The occurrence of silicate of alumina is a by no means rare 

 phenomenon in sandstone rocks. M. Etallon, in a short paper on 

 the soil of the Ores Bigarre, near Luxenil, in the department of 

 the Haute Saone, in France (and published there), describes the 

 association of alumina with silica and iron. The clay is a charac- 

 teristic feature of this lower member of the Trias. But 

 frequently silica has been introduced subsequently to the deposit, 

 and he suggests its origin in mineral springs. He mentions also 



