﻿286 ME. J. V. ELSDEisr ON THE [May 1908, 



group. With the more favourable specimens now available this 

 conclusion has proved to have been wrong, for the mineral is biaxial. 

 Dr. J. S. Plett has very kindly compared my specimens with still 

 more favourable examples found by him in another locality. 

 Although many of the physical characters point to prehnite, a 

 chemical analysis will probably be necessary before a definite 

 determination can be made (see PI. XXX, fig. 6). 



Pyroxenes. — Next to the felspars the pyroxenes are the most 

 abundantly represented, and their characters are remarkably uniform 

 wherever they occur. They are absent from the aplite-veins. 

 Phombic pyroxene is usually present, sometimes in the form of a 

 ferriferous enstatite, with the pleochroism of brouzite ; but in many 

 cases it is represented by bastite-pseudomorphs (see PI. XXXI, 

 fig. 2). It also seems to be occasionally changed to a fibrous trans- 

 parent pseudomorph of high birefringence, apparently one of the 

 amphiboles. A few sphene-granules are sometimes enclosed in the 

 rhombic pyroxenes. Enstatite generally crystallized later than the 

 felspars, by which it is often penetrated. Generally it clearly 

 preceded the monoclinic pyroxene ; occasionally it, as certainly, 

 followed that mineral, and encloses it (see PI. XXX, fig. 2) ; while 

 often the two forms are crystallographically intergrown, as was 

 also observed by Dr. Teall in the Whin-Sill rock.^ Here and there 

 are twins, one component of which is bronzite and the other augite 

 (see PI. XXXII, figs. 1, 2, 3, & 5). In this case the plane of com- 

 position appears to be the orthopinacoid of the latter. The variable 

 sequence of crystallization, and the frequent iutergrowths with 

 augite seem to lend support to the views of Prof. Yogt, who has 

 shown that upon theoretical grounds, and on the assumption that 

 the crystallization-curves of the ferriferous rhombic and monoclinic 

 pyroxenes belong to Poozeboom's Type V, the sequence of these 

 minerals should be dependent upon the relative proportions of lime 

 and magnesia present in the magma.^ In this case the simultaneous 

 separation of these minerals, as shown by iutergrowths and com- 

 pound twins, might represent the eutectic composition. The 

 monoclinic pyroxene is pale-brown in thin section. ITsually it is 

 remarkably free from alteration, showing no uralite-fringes, and 

 possessing the birefringence of a perfectly- fresh ferriferous augite, the 

 maximum value of y-a being about 0*022, as tested by Dr. Michel 

 Levy's colour-diagram. The maximum extinction-angle is about 40°. 

 No pleochroism has been detected. The most characteristic 

 inclusions are numerous rounded fragments of a paler pyroxene, 

 generally altered to greenish pseudomorphs. These seem often to 

 be optically oriented parallel to the c axis of the crystal : to these 

 I shall refer again. There are at least two distinct varieties of 

 augite, distinguished by the presence or absence of a well-marked 



1 Quart. Journ. Geol. Soc. vol. xl (1884) p. 649. 



2 ' SiKcatschmelzlosungen ' pt. i (1903) p. 129 & ibid. pt. ii (1904) p. 109 

 (Vidensk. Selsk. Skrifter Ohristiania). 



