REPORT OF THE CHIEF ASTRONOMER 441 



SESSIONAL PAPER No. 25a 



In the JSTorm classification the rock enters the dosodic subrang, yellowstonose, 

 of the alkalicalcic rang, coloradase, in the persalane order, britannare. 



During the metamorphism which even this type specimen has suffered, some 

 of its basic matter has probably been removed. The silica is thus believed to be 

 slightly higher than it would be in an analysis representing the original average 

 rock — perhaps by as much as four or five per cent higher. In the older classifi- 

 cation the rock is a granodiorite verging on quartz diorite. 



Along the eastern contact of the batholith the average plagioclase is labra- 

 dorite, A\ An u and it so far replaces the orthoclase that the rock becomes a true 

 quartz diorite. In the hand-specimen this somewhat basified contact phase is 

 indistinguishable from the true granodiorite. The limits of the orthoclase-poor 

 zone were therefore not closely fixed in the field. It is probable that the zone is 

 not more than a few hundred yards in width, and that the original rock of the 

 batholith was, in the large, homogeneous. A second exceptional phasal variation 

 is founded on the disappearance of hornblende in rock that shows decided cata- 

 clastic structure, other constituents remaining the same as in the normal grano- 

 diorite. This phase — gneissic biotite granite rich in andesine — occurs sporadi- 

 cally in the heart of the batholith. Possibly it is not of original composition, 

 the hornblende having been removed through metamorphic action. 



Dynamic and H ydr other mal Metamorphism of the Granodiorite. — Superim- 

 posed upon the original variations in the batholith are the much more striking 

 effects of intense orogenic strains. Even the most massive phases show, under 

 the microscope, the varied phenomena of crushing stress — granulation, bending 

 of crystals, undulatory extinctions, and recrystallization. Because of the crush- 

 ing, the average rock is no longer the original rock. The granodiorite has been 

 changed into several metamorphic types, of which three may be noted. 



The commonest transformation is that into a hiotite-epidote-hornblcnde 

 gneiss, with essential and accessory constituents like those in the original grano- 

 diorite, but in somewhat different proportions. The colour is light gray, with a 

 green cast on surfaces transverse to the schistosity; parallel to the schistosity a 

 dominant and darker green colour is given by abundant fine-textured leaf aggre- 

 gates of biotite. These aggregates are not simply crushed and rotated original 

 mica foils, but, like the epidote, represent true recrystallization and the incipient 

 migration of material within the granulated plutonic rock. At the same time 

 much of the original hornblende, apatite, and magnetite have been removed. 



A second metamorphic type is a yet more highly schistose hiotite-epidote 

 gneiss often transitional into biotite schist. The essential constituents are 

 biotite, epidote, orthoclase, andesine, and quartz. The accessories include very 

 rarely apatite and magnetite, while titanite seems to have entirely disappeared 

 along with the hornblende. Orthoclase seems here to be more abundant than 

 plagioclase. The quartz and feldspars are intensely granulated and, with polar- 

 ized light, are full of strain shadows. The rock is more richly charged with 

 biotite than the hornblende-bearing gneiss. 



The third metamorphic type occurs in immediate association with the gneiss 

 just described, being interbanded with it. It is a fine-grained, strongly schistose. 



