54 Diarizes and Granites of Swift's Creek, 



the intrusive masses near Odell's Creek, going north and 

 east round to near Sand} 1 - Gully Gap. Here, however, 

 the plagioclase is of such extreme rarity that its presence 

 but proves the general rule as to its absence. Where it 

 occurs it is in well-formed, prismatic crystals, resembling those 

 of the granites. I was not able to obtain any measurements. 

 Felspars occur in the hornfels zone so rarely that it may be 

 stated as the rule that they are not found there at all. The 

 only instances I met with were in the actual contacts of the 

 hornfels and quartz diorite. From the best example which 

 I collected, which was taken from the adit of the Black 

 Prince Mine, at Eureka, I was enabled to prepare a slice 

 across the contact of the two rocks, the hornfels being in 

 this case a very quartzose example. A number of granules 

 and some perfectly-formed crystals of felspar are situated 

 among the granules of quartz near the contact line. The 

 position of these felspars, only when close to the dioritic rock, 

 and their mostly sharp and well-marked outlines, proved 

 that they are secondary, and not original, constituents of 

 the sedimentary rock. The largest felspar crystal is com- 

 posite as to one half and simple as to the other, the lower 

 part of both halves being very homogeneous, except where 

 crossed by a few laminae, The angle of obscuration is not the 

 same in the upper and the lower parts, being in the former 

 14° and in the latter onry 11°. From the structure and 

 optical characters, I think this felspar belongs to group 1 

 (albite or oligoclase). Besides the large crystal, there are 

 other triclinic felspars in mere granules, in which no 

 measurements could be obtained. 



In the aplite bands which lie nearest to the intrusive 

 masses there is, beside the small constituent amount of 

 triclinic soda and soda lime felspars, which I have already 

 mentioned, a very large amount of triclinic and a lesser 

 amount of monoclinic potash felspars. 



The triclinic potash felspar is crystalline granular, and 

 nowhere well crystallised. It has shades of pale flesh red to 

 white. In order to test the presumption raised by my first 

 examination of some thin slices of these rocks, I broke up an 

 average piece of the rock, and by aid of the lens selected a 

 number of the minute flakes which had been separated from 

 the most perfect cleavage. Some of these I examined 

 qualitatively before the blowpipe, and others I prepared as 

 microscopic objects ; the former gave (bb) a slight sodium 

 and a strong potassium flame reaction, and tested in the wet 



