GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 451 



mense veins of lighter coloured rock, reaching four feet 

 thick, nearly white where fresh, but generally light greyish- 

 green from decomposition, traverse the main mass, and 

 catch the eye upon the walls of the quarries. These are 

 aplitic veins, the final product of differentiation resulting 

 from the cooling of the mass. Less obvious, but equally 

 interesting, evidence of differentiation by other processes 

 preceding the aplitic segregation has been obtained by 

 Messrs. Jensen and Jevons in the course of their study of 

 the proportional distribution of the minerals in different 

 parts of the mass, and will be stated in the second part of 

 this paper. An interesting series of decomposed products 

 is found, and analcite occurs in such manner as makes it 

 necessary to discuss the possibility of its being of primary 

 origin. 



Exposures of the igneous rock are numerous and good. 

 By far the best section is given by the Old or Reservoir 

 Quarry, situated on the westernmost slope of Prospect 

 Hill, close to the eastern end of the great dam for which 

 it provided the massive stone facings. As shown by the 

 map (Plate XXXIV), a level floor has been cut nearly one 

 hundred yards back into the hill, thus exposing a steep 

 freshly cut face of rock three hundred yards long, by from 

 seventy to eighty feet in height. A sketch of part of its 

 face is given in the transparent sheet overlying Plate 

 XXXV, which is a photograph of a part of its face. Other 

 exposures on the W. and S.W. of the mass are afforded by 

 some shallow workings a few yards S.E. of the south end 

 of the Reservoir Quarry, and by Booth's Quarry a little 

 further to the S.E. These afford excellent sections of the 

 outer envelope of the intrusion and of the overlying shales. 

 A little to the north of the centre of the mass, lies the 

 Emu Quarry (so-called because it has been for some years 

 worked by the Emu Plains Stone and Gravel Company). 



