131 



in general rather fresher than the orthoclase, which occurs 

 in large allotriomorphic grains, untwinned, and full of dusty 

 inclusions. Both felspars are full of inclusions of uralite, 

 magnetite, etc., and have somewhat decomposed with the 

 production of kaolin. Quartz also occurs in smaller irregular 

 crystals. The base is finely granulitic. It consists of quartz 

 and orthoclase, the latter recognizable by its low refractive 

 index, irregularity of outline, and slight decomposition — the 

 former, which is predominant by its more rounded outline, 

 greater clearness, and, where large enough, by its unaxial 

 figure. It is difiicult to say whether or not plagioclase is pre- 

 sent in the base. One would expect it to occur, but the 

 absence of twinning, so common in granulitic plagioclase, 

 smallness of size, and the fact that the refractive index is 

 nearly equal to that of quartz (certainly greater than Canada 

 balsam) makes identification very difficult, though some grains 

 suggest plagioclase rather than quartz. Uralite, after diop- 

 side, is the predominant ferromagnesian silicate, and occurs 

 in the same manner as in slide 604 (Aldgate), though much 

 smaller. With it is associated a good deal of magnetite, and 

 the distribution of the two minerals indicates slightly the 

 schistosity of the rock. Mica occurs both as muscovite and 

 as biotite, the former in comparatively large, clear plates, the 

 latter in small pleochroic brown plates. Small-rounded grains 

 of rutile and a little apatite, as well as the secondary mine- 

 rals haematite and leucoxene, are also present. Name. — 

 — Quartzose felspar porphyry. 



In three particulars this rock shows itself allied to those 

 of the Houghton magma : — 



Ui) Acidity of the plagioclase. 



(h) Nature, mode of occurrence, and alteration of the 

 predominant ferromagnesian silicate. 



(c) Presence of titanium. 

 For these reasons it seems justifiable to consider this rock 

 as an effusive product of the Houghton magma, so that not 

 only was the Houghton magma active in producing Pre-Cam- 

 brian plutonic intrusions, but also volcanic lavas were pro- 

 duced. There is at present no evidence of the seat of this 

 volcanic activity, though it was probably south of Jamestown, 

 the drift of the Cambrian till-forming ice being apparently 

 northerly. (21) 



via. Olary. 



The aplite referred to previously is figured on plate iv.,. 

 fig. 9. It is pink in colour, with a grainsize of about 2 mm., 

 quartz felspar and muscovite being recognizable macrosco'pi- 



(21) W. Howchin, Q. J.G.S. (Ixiv.), 1908, p. 258. 

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