and their Contact Zones. 27 



whose optical sections do not form angles above 20° in the 

 zone o — ii, the probability becomes strengthened that the 



former are sections of oligoclase crystals taken parallel to ii. 



In the measurement made of these felspars I find the 

 angles 0°, 6°, 13°, 15°, 17°, 18°, 20°, to be of most frequent 

 occurrence. 



It is of interest to note the association of other mineral 

 constituents with this group, and on this point I have 

 endeavoured to gain information by carefully tabulating my 

 data. The results are, of course, only approximate, and as 

 such I give them ; in fact, merely as suggestive of the pro- 

 bable truths. 



I selected thirty-two slices which I regarded as fairly 

 representing the crystalline granular igneous rocks collected. 

 But in this collection I must note that the granitites and 

 amphibole granites are in a minority, as they did not attract 

 my attention so strongly by their contact phenomena or 

 their auriferous character as did the diorites. Of these I 

 found four in which the predominant felspar came within 

 the requirements of this class. With it were associated 

 subordinately labradorite, mica or hornblende, and quartz. 



2. Labradorite and Anorthite* 



■ These felspars are found most commonly in the felspathic 

 diorites along the south margin of the intrusive area in the 

 Gum Forest ; especially, however, at Riley's Creek and at the 

 Sheep Station Creek Gap.; They are usually associated with a 

 little hornblende, or mica, or both, and more rarely with 

 quartz. 



Felspars, whose angles of obscuration point to albite or 

 oligoclase, are entirely subordinate where these occur, except- 

 ing in one locality near Eureka where I have met with 

 rocks in which the felspars of this group and the former 

 were in nearer proportion. 



As the more acid felspars of group 1 increase, so does the 

 percentage of quartz increase in amount in the rock. I 

 have only observed two instances where felspars having the 

 optical angles of this group occurred in company with a pre- 

 ponderance of microcline or of other tricline felspars. 



The compounding and general structure of these felspars 



* Andesine has been omitted, but there are measurements which cover 

 that species. 



' D2 



