GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 521 



essexite on the sides of the mass has the same composition 

 as on the top, and if so there must be horizontally the same 

 change on going inwards to the main rock as was found in 

 going vertically downwards; but there is nowhere any 

 indication of further horizontal change. 



16. Metamorphism and Assimilation of the Country Rock. 

 The only kind of country rock abutting on the intrusive 

 rock at the present surface of the ground is the Wianamatta 

 shales, which varies little in composition, except that 

 occasional bands are more sandy and others more calcareous 

 than the average. In no place is more than the very 

 slightest alteration apparent. The shales have been con- 

 siderably hardened, enabling them better to resist weather- 

 ing, but in the hand-specimen no other change but the 

 different fracture due to this hardening is visible. In thin 

 section under the microscope, specimens of shale taken 

 from close to the junction appear precisely the same as 

 unaltered shale, so that the alteration which has taken 

 place must be extremely slight, and be confined probably to 

 a recrystallisation on an extremely minute scale. Optical 

 investigation having given this negative result, the matter 

 did not seem worth pursuing by chemical analysis. 



Melting up and assimilation of the country rock has 

 undoubtedly occurred to some extent, though probably not 

 on a large scale. Direct evidence is afforded by the occur- 

 rence in a number of slides of the pallio-essexite of circular 

 or oval patches which are nearly opaque or dark grey in 

 colour, having a minutely granulated appearance ; do not 

 depolarise except weakly as a very fine mosaic under 

 the high power; and whose edges are not defined, but shade 

 away gradually into the surrounding fine-grained ground 

 mass. These are unquestionably partially absorbed frag- 

 ments of shale, which they closely resemble in every way 

 under the microscope; but no special minerals are seen 



