138 



simple individuals, or binary twins, was regarded, for a long time, as sufficient 

 evidence of its existence. It is now known that these characters are 

 frequently possessed by the different varieties of plagioclase. The most 

 valuable tests for the determination of orthoclase in thin sections are (1) straight 

 extinction in sections giving rectangular outlines, that is in sections of the 

 zone 100 : 001 ; and (2) simultaneous extinction in the two halves of a binary 

 twin (Carlsbad) when the trace of the face of composition lies parallel with the 

 short axis of one of the nicols. The latter character shows conclusively that 

 the section is one of orthoclase cut at right angles to the clinopinacoid 

 (010). Of course many sections of Carlsbad twins of orthoclase split up under 

 crossed nicols into two halves which do not extinguish simultaneously, but these 

 never show symmetrical extinction with reference to the trace of the 

 composition-face. It therefore follows that symmetrical extinctions in sections 

 which show normal twinning, whenever they occur, imply that the felspar is 

 triclinic. 



Chemical analyses show that potash, in small quantities, is a normal 

 constituent of basic rocks. This fact does not, however, prove the independent 

 existence of a potash felspar, as the substance may enter into the composition 

 of a triclinic species belonging to the soda-lime group. Indeed, chemical 

 analyses and flame reactions constantly show the presence of a small amount of 

 potash in the soda-lime felspars. Although we cannot at present speak 

 positively as to the occurrence of orthoclase in the ordinary basic rocks of 

 Britain, there can be no doubt that it exists in the so-called contemporaneous 

 or exfiltration veins in such rocks. Thus, Mr. WALLER has shown (1) that 

 certain red and grey veins in the Rowley Rag dole rite are mainly composed 

 of felspar occurring as simple individuals or binary twins, and that the two 

 halves of the latter often extinguish simultaneously when the length of the 

 section and the trace of the composition- face are parallel with the short axis 

 of one of the nicols. He has further proved by chemical analysis that the rocks 

 of these veins contain a much higher percentage of silica (58 as against 

 48 p.c.), and also a much larger amount of potash. We have now to consider 

 the dominant felspars of the basic group of rocks. These may be either 

 primary or secondary ; the latter, however, are only known in rocks that have 

 been affected by contact or regional metamorphism. The primary or original 

 felspar may occur (1) as porphyritic crystals with more or less definite form, 

 or as fragments of such crystals, (2) as porphyritic granular aggregates, (3) as 

 small columnar crystals, giving lath-shaped sections, (4) as irregular grains, 

 and (5) as skeleton crystals and microlites. 



The porphyritic crystals are usually somewhat tabular in form, with 

 conspicuous development of the brachypinacoid, M (010). The other common 

 faces are P (001), 1 (110), T (110), and x (fOl). The curious form which is 

 characteristic of the rhomben-porphyr of Norway, in which the faces T, 1 and 

 y (201) are alone developed, is not known in any British rock, and seems, 

 indeed, limited to rocks exceptionally rich in alkalies. 



The porphyritic and other felspars of the rocks under consideration are 

 usually twinned. The most common type of twinning is that known as the 



(1) Midland Naturalist. Vol. VIII. 1885, p. 261. 



