Ordovician Sandstone. 6? 



many of the twinned felspars may be anorthoclase, as will be 

 indicated later, point to the probability of the rock being a soda- 

 rich granite similar to that at Station Peak. In fact, it is 

 remarkably like this latter, often containing large phenocrysts 

 of felspars although perhaps not as large as the You Yangs 

 specimens. 



Numerous veins of aplite and tourmaline aplite traverse the 

 granitic mass, and also run out into the Ordovician at the con- 

 tact. It is, however, with the method of intrusion and the dif- 

 ferentiation of the Harcourt plutonic magma that this paper is 

 more directly concerned. 



The accompanying photograph is of an inclusion of country 

 rock in the granite near Big Hill. This specimen occurs on the 

 south slope of Big Hill, in the bed of a smaU tributary to Bullock 

 Creek, and is situated at least 200 yards from the contact of 

 the intrusion with the Ordovician. At this point the tributary 

 has exposed the bare surface of the granite over an area of 

 a few square feet, and has rounded and smoothed the rock sur- 

 face considerably. As will be noted from the photograph, the 

 inc'usicn (A) stands up in relief from the granite surface (C). 

 Surrounding the inclusion, except for two inches on the right- 

 hand side, is what appears to be a basic segregation (B) from 

 the hand specimen, and this latter, in contrast to both Ordovican 

 inclusion and surrounding granite has been eroded to a maxi- 

 mum depth of three inches below the granite surface. Two 

 very thin, light-coloured veins or threads cut through both the 

 country reck and segregation, and apparently run into the granite 

 at the side where the granite is in actual contact with the sand- 

 stone inclusion. The original sandstone has been altered to a 

 ronsiderable extent. 



Description of specimens : — 



Granite, Big Hill. — A light grey apparently normal granite 

 of quartz^ felspar and biotite, often containing fairly large crystals 

 of felspar. Under the microscope shows typical granitic texture. 

 Abundant quartz and orthoclase in large allotriomorphic crystals, 

 twinned and zoned felspars ranging from oligoclase to albite 

 in smaller crystals, but relatively abundant and approximately 

 equalling orthoclase in amount. Owing to the extremely minute 

 twinning of some of the felspars, they may be possibly anortho- 

 clase. In two or three of the large orthoclase crystals extremely 

 thin lamellae can be just barely detected, pointing to a possible 



6a 



