26 Diorites and Granites of Swift's Creek, 



1. Albite and Oligoclase. 



Rocks containing felspars, whose angles of maximum 



obscuration in the zone o — ii, as against the edge o — ii, point 

 to the above, are found at Lower Riley's Creek, near the 

 Long Gully Gap, at Upper Swift's Creek, and at Eureka. 



The principal point to be noted in these felspars is that 

 they are usually compounded of very numerous narrow or 

 alternately narrow and wide laminae. They occur either as 

 well-formed prismatic crystals having plane or compound 

 terminations, or of much larger size, in which case they are 

 less well denned. 



As to the mode, of composition I note as follows : — 



1. Simply compounded of many alternating laminae 

 (albite law). 



2. Doubly compounded, each half being as in 1, composed 

 of many twin laminae, or one-half thus compounded, while 

 the other half (right or left) is plain. In this case we may 

 suppose that the compounding has followed two laws — first, 



that of " axis of rotation normal to ii ;" and, second, that of 



" axis of rotation parallel to ii ;" or, to use expressions which 

 will be very convenient for the purposes of this essay, 

 according to the " albite" and " carlsbad" laws. More 

 rarely there are cases in which the crystals consist of two 

 pairs of twins compounded as above, thus forming a quartet. 

 A third law may there be regarded as being followed — 

 namely, the " pericline law/' where the axis of rotation may 

 be considered as the macrodiagonal. 



The irregularities of twinning consist principally in the 

 presence of isolated laminae, or of small associated twin 

 laminae in otherwise plain sections. In some instances 

 there are also single laminae across the general composition. 

 In other infrequent cases I have observed a twin dissimi- 

 larly compounded, having, for instance, the right half plain 

 and the left half twinned (pericline law) in a direction per- 

 pendicular to the composition face. 



Together with these undoubtedly triclinic felspars there 

 are many sections which show no trace of twin composition. 

 Some may be orthoclase, but there are others whose outlines 

 are not rectangular but rhomboidal, and yet in which 

 the direction of greatest obscuration is parallel to the 

 larger side. This, I think, directly points to oligoclase; 

 and where such sections are associated with twinned felspars 



