•52 Diorites and Granites of Swifts Creek, 



sections are moderately diehroic. It is not fibrous, but yet 

 not glassy, as are many of the hornblendes of the diorites. 

 It takes up most of the space in the slice. Next to it in 

 •a mount is a colourless and not diehroic pyroxene, either 

 separately or grouped, of several more or less perfectly 

 prismatic crystals. The characteristic twinning of augite 

 occurs. This being an augite-like pyroxene, with very little 

 iron, is probably diopside. The angles of maximum obscura- 

 tion of the amphibole and pyroxene I found to be as high as 

 16° 39' in the former and 44° in the latter. The most 

 interesting feature in this rock is, however, the constant 

 intergrowth of amphibole and pyroxene. In Fig. 20 I give 

 an example. 



In another instance in the same slice the pyroxene formed 

 the nucleus of the amphibole crystal. The alterations which 

 I observe in this slice have been principally the production 

 -of an almost colourless chlorite, otherwise precisely similar 

 in character to thatT have described when speaking of the 

 diorites as the result of alteration of the hornblende. It is 

 rather broadly fibrous, undulating, and only faintly diehroic 

 in shades of green. Iron ores have separated, and are 

 deposited in the chlorite or in masses, either as magnetite or 

 ferric oxide, and in the latter case it is in red and trans- 

 lucent flakes. I have also observed the incipient formation 

 of chlorite as minute acicular fibres fringing the margins of 

 the amphibole. 



No felspars are recognisable ; but if any existed they may 

 be now represented by some small spaces filled by minutely- 

 constituted, brightly-polarising aggregates. No quartz is 

 met with in the slices. 



In another sample obtained from a dyke in the neigh- 

 bourhood of the last, I found the amphibole to be in prisms 

 much drawn out in the direction of the chief axis, and 

 prismatic cleavage well marked in the cross-sections. The 

 angles of obscuration in these amphiboles are large, being, 

 according to my measurements, as high as 21° 30'. Many of 

 the smaller crystals are twinned according to the ordinary 

 law. These crystals form at least two-thirds of the rock, 

 the remainder consisting of alteration products, among which 

 is the usual chlorite, and also the minute aggregates spoken 

 of in the last. In this slice, however, there are also a number 

 of long and narrow plagioclase prisms in all stages of altera- 

 tion, and connecting the scarcely altered individuals with 

 the resulting minute aggregates before spoken of. In the only 



