Vol. 70.] COMPOSITION OF ROCKA.LLITE, 295 



Survey. For the analysis about 8 grams were available, a few 

 grams being reserved for optical study and contingencies. I would 

 here express my thanks to Dr. F. E. Wright for kindly undertaking 

 the special optical study of the pyroxene. 



Petrographical Description. 



The texture of the rock is granitic, somewhat] porphyritic in the 

 Inskip specimen according to Prof. Judd, although this was not 

 very evident in my small piece, but non-porphyritic in the other two. 

 It is fine-grained (millimetre-grained), and composed of small 

 anhedra of colourless quartz, white felspar, and thick prisms of a 

 greenish-black pyroxene, the three being present in about equal 

 amount. These three minerals are all perfectly fresh, but there 

 are small specks and patches of a powdery, pale yellow-brown, 

 evidently ferruginous, decomposition -product, apparently limonitie. 

 The water yielded by the analysis shows that the amount of this 

 cannot be great. The specific gravity of this specimen is stated by 

 Prof. Judd to vary between 2*92 and 2*94, that of the other two 

 specimens being 2*77 and 2 - 71 respectively. 



In thin section the rock is seen to be composed almost entirely 

 of sodic pyroxene, alkaline felspar, and quartz, the pyroxene having 

 crystallized first, then the felspar, and last the quartz, as noted by 

 Judd. The pyroxene is in subhedral prisms, with no very definite 

 terminations, and only seldom showing prism or pinacoid faces. 

 Some small subhedral crystals are also included in the quartz 

 and felspar. Cleavage is well-marked, and the crystals carry no 

 inclusions. 



The pyroxene is of two kinds. A bright grass-green aegirite is 

 abundant. The extinction -angle (Wright) varies from 3° to 5 . 

 and the rather strong pleochroism is, as determined by Judd : 

 a grass-green, 4) pale olive-green, c yellow-green, with the ab- 

 sorption a^b>c The refractive indices, as determined by 

 Wright, are a=about 1*765, y >1'80<1'S2. This green aegirite 

 forms usually the inner portions of the larger prismoids and most 

 of the smaller, isolated, euhedral crystals. The other variety of 

 pyroxene is a pale yellowish -brown a c mite, with extinction -angles 

 slightly greater than those of the aegirite. Its pleochroism is not 

 so strong ; a brown, u yellow-brown, c brownish yellow. Its re- 

 fractive index is practically the same as that of the aegirite, but 

 possibly a little higher, and, as Dr. Wright says, the difference 

 optically between the two forms is not so pronounced as one might 

 be led to expect from the difference in colour. This brown acmite 

 generally surrounds the green aegirite, the border between the two 

 being rather irregular but fairly sharp. Rarely the aegirite is 

 exterior to the acmite, especially at the ends of prisms, and there 

 are a few small crystals composed entirely of acmite. Cleavage 

 does not seem to be as well developed in the acmite as in the 

 aegirite. 



The felspar occurs, for the greater part, as comparatively large, 



Q. J. G. S. No. 278. x 



