50 Diorites and Granites of Swifts Creek, 



the whole mass. Quartz is rather plentiful in small clear 

 grains, which, however, often suggest that they are 

 secondary. Throughout these and throughout the whole mass 

 are very many minute apatite needles. Ores of iron have 

 been deposited besides those magnetite crystals, which may 

 be regarded as original. 



Crystalline Giximdar Diorites. — The dykes of this variety 

 are found occasionally throughout the district, aud are 

 usually strong and persistent. One of them traverses the 

 schists on the eastern boundary. As usual, the thin slice 

 showed me that the rock has undergone much alteration. 

 The main mass is made up of more or less imperfectly 

 formed, but rather stout, prisms of triclinic felspars, 

 all of which, with few exceptions, are completely 

 altered into aggregates of some minute micaceous 

 mineral. In none of those which still showed striations 

 could I obtain measurements. Between these felspars there 

 have been small, elongated prisms of hornblende, all now 

 converted into strongly dichroic chlorite. Whatever little 

 ground mass there was has become involved in the general 

 micaceous and chloritic alterations. A little quartz rills in 

 spaces. 



Another and somewhat similar dyke traverses the edge 

 of the metamorphic schists in the same neighbourhood. It 

 runs for two or three miles, and varies in texture. I pre- 

 pared samples taken from the most crystalline granular part 

 of this dyke, showing the apparently least altered rock, and 

 also examples from a little distance, where the alteration 

 seems to me to be at a maximum. I found a thin slice 

 of the former to be composed as follows : — (1.) Felspars 

 numerous, and in well-defined prismatic crystals, both singly 

 and doubly compounded. The angles of obscuration 

 measured gave me from 15° to 23° 30' on either side of the 

 composition face, and this would, I think, point to labrado- 

 rite. The felspars are all more or less converted into 

 aggregates of some minute micaceous mineral (soda or lime 

 mica ?). The inclusions are apatite and, rarely, small flakes of 

 hornblende. (2.) Hornblende, in flakes or imperfectly formed 

 crystals, showing the prismatic cleavage of 124° 30'. They 

 are frequently twinned, and are strongly dichroic. They 

 include portions of ground mass and magnetite. The horn- 

 blende has been in places converted into chlorite, with the 

 production of iron ores. In another slice the hornblende 

 was replaced by brown dichroic magnesia mica. Quartz fills 



