328 Ernest W. Sheats : 



Professor Gregory agreed with Dr. Hewitt in the naming of the 

 p jrphyi'ite, but regards fine^-grained grano-diorite as being a more 

 suitable term for the rock which Dr. Howitt has referred to the 

 aplites. 



I can appreciate the difficulty in finding an appropriate name 

 f jr this type, and I think this difficultj' is connected with its mode 

 of origin. The minerals present are plagioclase (oligoclase to 

 labradorite), hornblende, biotite, a white mica, quartz and 

 orthoclase in the rock (No. 627) from just S.W. of Gate 45, S. 

 HeaihcO'te, while in the Red Hill exposure very little hornblende 

 or biotite occurs (No. 587), but the rock is more altered, and 

 they may have been removed by solution. I think that perhaps 

 the term micro-granite, using it in a wide sense', would be an 

 appropriate name for most of the occurrences, while many of 

 the rocks show so marked a granophyric or micrographic inter- 

 growth of quartz and orthoclase that the term granoph}Te may 

 be used to define these varieties. 



The origin and relations of these rocks merit some considera- 

 tion. They are certainly not normal plutonic rocks. The fine- 

 grained character and other peculiarities show that the magma 

 consolidated under a comparatively small thickness of rock, and 

 its mineral composition and the micrographic and granophjTie 

 intergrowth of the quartz and orthoclase suggest that it repre- 

 sents a residual relatively acid, part of a basic magma, and having 

 more or less the oomposition of an eutectic mixture it remained 

 molten after the intrusion and consolidation of the more basic 

 part of the magma. I regard these micro-gTanites and grano- 

 phyres and the labrador porphyrite as genetically related to the 

 diabases, belonging to the same volcanic period, and represent- 

 ing the last of a series of intrusions, of which the diorites mark 

 an earlier phase. 



The microgranite in places comes into relation with the 

 Ordovician series, although no good contact has been observed. 

 Of course, if the granite were the older series no contact meta- 

 morphism should be seen. Even if the gi-anite were intrusive 

 into the Ordovician the amount of alteration to be expected at 

 the junction would, I think, be very small in view of the probable 

 relatively superficial conditions and low temperature of its consoli- 

 dation. The contact of diabase and micro-granite is well exposed 



