30 Diorites and Granites of Swift's Creek, 



observed the striated part to surround a central curiously- 

 mottled and unstriated space. In this case I have noted a 

 cross-banded structure, which is of common occurrence. I 

 give in Fig. 15 a well-marked instance of this characteristic 

 mode of composition. 



These felspars do not, as a rule, polarise with those bril- 

 liant colours which are seen in groups 1 and 2 ; but on being 

 rotated between crossed nicols the unstriated and mottled 

 portions, or what may be said in some cases to be the mass 

 in which some few twin lamellae are situated, shows a 

 peculiar bluish or brownish satiny sheen, which I have 

 found to be highly characteristic of these felspars. 



Cleavage is not often observable with any distinctness in 

 these felspars, and their physical structure is otherwise 

 peculiar. Many cases occur where they are fractured — the 

 parts separated, and perhaps cemented together by ground 

 mass. As a rule, they are well preserved, and rather glassy 

 in appearance. 



The only inclusions which I have observed are apatite 

 prisms, and needles. The alterations are not great, and have 

 principally produced kaolinisation, and only more rarely 

 mineral aggregates of the mica group. Many of the minute 

 cracks traversing these felspars are the depositories of infil- 

 tration products — such as ferric-hydrate. 



There are, therefore, among these felspars those which are 

 wholly twinned, or partly twinned, and those which are 

 simple. The angles of obscuration referred to the composi- 

 tion face in slices approximately in the zone o ii, when the 

 individual is compound, are under 18°, and most frequently 

 14°, 15°, 16°, while in the simple portions the direction of 



obscuration often conforms with the edge o — ii, supposing the 



crystal to be triclinic or o — ii, supposing it to be monoclinic. 

 These angles do not agree with the triclinic soda and soda 

 lime felspars, nor with orthoclase. 



Des Cloizeaux has pointed out* that the triclinic potash 

 felspar microcline is sharply marked by its characters. A 

 slice parallel to the basis shows the direction of obscuration 



inclined to the edge o — ii at 15° — 16°, instead of being parallel 

 to it, as in orthoclase. Many of the microcline group — as, for 

 instance, Amazonstone — are distinguished by two systems of 



* Comptes Bendus, Ixxxii., No. 16 ; also see Rosenbusch Mih'oskopische 

 Physiographie der Massigen Gesteine, 1877, p. 14. 



