190 R. A. Daly— Secondary Origin of Certain Granites. 



rarely is feldspar essential ; in one slide it seems to compose 

 ten to fifteen per cent of the rock. So far as observed, the 

 feldspar of the staple quartzite is orthoclase. No sodiferous 

 mineral has been certainly determined in the rock. Epidote, 

 zoisite, titanite, magnetite, leucoxene, pyrite and zircon, besides 

 chlorite, secondary after biotite, are the other, always subordi- 

 nate, constituents. 



In marked contrast to the normal quartzite is the rock col- 

 lected at a point thirty feet from the upper contact of the 

 Moyie sill. It is a very hard, vitreous, massive, light bluish 

 gray quartzite carrying much feldspar. The whole rock 

 seems to have been recrystallized. The granular-mosaic struc- 

 ture has been largely replaced by poikilitic and micrographic 

 structures. Quartz is thus either regularly intergrown with 

 feldspar or else encloses non-oriented individuals of the same 

 mineral. The feldspar proved to be orthoclase, albite and 

 microperthite, named in the order of their relative abundance. 

 Biotite and sericitic muscovite are, as usual, in considerable 

 amount. A little magnetite and a few minute crystals of ana- 

 tase are the subordinate minerals. The characters of this con- 

 tact phase point to the thorough metamorphism and notable 

 feldspathization of the quartzite in the external contact zone 

 of the gabbro. 



The main mass of the sill-rock has the composition noted 

 above as found in the sills generally. The grain is here 

 medium to coarse, the structure hypidiomorphic-granular. 



At the lower contact the grain of the gabbro is somewhat 

 finer than in the interior of the sill, but the rock is still 

 medium-grained and never compact. At the same time, inter- 

 stitial and poikilitic quartz, along with biotite, orthoclase and 

 microperthitic feldspar, are increased in amount. There is 

 thus some acidification of the sill at its lower contact, though 

 the rock is still gabbroid in macroscopic appearance and has 

 hornblende and plagioclase (andesine to labradorite) as the 

 chief constituents. Acidification of this order is visible for at 

 least 200 feet from the lower contact. The intrusive rock is 

 yet more abundantly charged with quartz, biotite and alkaline 

 feldspars in the vicinity of the occasional xenoliths torn from 

 the invaded quartzites. 



The conditions are different at the upper contact. They 

 may be readily studied on the wagon-road that threads the 

 floor of the western meridional valley, shown in fig. 1. From 

 the upper contact inward for a perpendicular distance of about 

 150 feet the intrusive is a highly siliceous rock, the mineralog- 

 ical composition of which is shown in Table I and Table II. 

 The structure of this rock varies irregularly, even in the same 

 slide, from the hypidiomorphic granular of granite to the 

 structure of granophyre or micropegmatite. 



