418 Transactions. 



is general, the optical character is negative, and the elongation positive, 

 whilst the optic axial angle is small. Most important of all, however, the 

 extinction, whilst often nearly straight, reaches generally as high as 5° 

 with the composition-planes of the lamellae, and in one or two crystals 

 angles of over 15° are got with the cleavage. Were it that only one or 

 two crystals showed the obliquity of extinction one might suspect some 

 inaccuracy of observation, but at least eight in the slide show it. Further, 

 even greater obliquity of extinction, with similar very finely microscopic 

 twinning, is shown by nearly a dozen crystals of apparently identical 

 hypersthene-like pyroxene in a noritic rock discovered at Milford, and 

 described later in this paper (see p. 420). 



In some instances the lamellae are here much broader than in others, 

 whilst the extinction angles generally range up to 15°, but in one case up 

 to as much as 42°. As a rule, the lamellae extinguish together in the one 

 direction, but this is by no means invariably the case. In the Milford, 

 as in the Cleddau-HoUyford rock, there is parallel intetgrowth with fine 

 lamellae of augite. It may be thought that the crystal in which the 

 extinction angles of the lamellae reached 42° exemplifies a similar parallel 

 intergrowth with augite, but this is not so, for the lamellae are very dis- 

 tinct and comprise the whole mineral, which certainly is not an ordinary 

 monoclinic pyroxene, for it is optically negative and has the distinct 

 hypersthene-hke pleochroism already noted. There is, however, one pecuh- 

 arity of this crystal that may have a bearing upon the matter — namely, 

 the optic axial angle appears to be large. 



Only one further observational fact now remains to be mentioned in 

 connection with this pyroxene, but it is of importance : the plane of the 

 optic axes coincides with the composition-plane of the lamellar twin. Since 

 the extinction is oblique, this plane must be at right angles to the plane 

 of symmetry — that is, to the 100 plane — as in normal hypersthene. The 

 lamellar twin appears not to follow the plane 110 frequent in the inter- 

 growths of rhombic and monoclinic pyroxenes, for all the sections showing 

 the lamination distinctly are approximately at right angles to the optic 

 axial plane ; nor is it the common 010 plane.* It is much more reasonably 

 the 100 plane observed by Bowen in artificial cUno-enstatite.t On this 

 supposition, however, it is difficult to understand why the lamellae often 

 appear all to extinguish in the same direction. 



Enough data are available to show that a monoclinic pyroxene of 

 unusual type is exhibited in the sections from the Cleddau-Hollyford and 

 the Milford rocks. It approaches hypersthene, but differs from it in its 

 obhque extinction, and it seems probable that it is a monocHnic member 

 of a series embracing this latter. It is obvious that much of the mineral 

 in the sections that is apparently hypersthene may really be this mono- 

 clinic pyroxene viewed in sections lying near the orthopinacoid. 



Chemical Classification. 



An analysis of the Cleddau-Hollyford quartz-norite by the Dominion 

 Analyst gave the results tabulated in column A below. In columns B 

 and C the analyses of somewhat similar rocks are quoted from Iddings.J 



* See H. RosENBUscH, Mikroskopische Physiographie der Mineralien und Gesteine, 

 vol. 1, pt. 2, p. 149, 1905. 



t N. L. Bowen, The Ternary System Diopside-Forsterite-Silica, Am. Journ. Sci. (4), 

 vol. 38, pp. 207-64, 1914. 



J J. P. Iddings, Igneous Rocks, vol. 2, p. 218, 1913. 



