and their Contact Zones. 49 



to the alteration of amphibole. The general character of the 

 rock, the proximity of the dykes to the intrusive anorthite 

 dioriteSj their non-occurrence elsewhere, point to their being 

 a plagioclase felsite. 



The fourth dyke examined is of a very finely granular, 

 approximating to a compact, texture. It has a crypto-crys- 

 talline ground mass of minute, doubly-refracting particles, in 

 which are — (1) Minute plagioclase prisms ; (2) iron ores 

 (magnetite and pyrite ?), and (3) a large amount of viridite 

 (chlorite ?). This rock evidently approaches in composition 

 those diorites of the immediate neighbourhood in which the 

 amphibole (here converted into viridite) preponderates over 

 the anorthite or labradorite felspars. This dyke might be 

 classed with diorite (aphanite). 



Diorites. — Dykes of this class are of very frequent 

 occurrence near the contacts, but are also found throughout 

 the whole intrusive area, with the exception of the tracts of 

 anorthite diorite and amphibole-gabbro, which they do not 

 penetrate. These dykes are from a foot to 5 feet in width, and 

 have not any common direction of strike. They vary much 

 in texture, from finely crystalline or almost compact to a 

 texture in which the crystalline individuals of white plagio- 

 clase and dark-green hornblende are clearly distinguishable 

 by the naked eye. In extreme cases the individuals may be 

 even *25 inch in diameter. I have observed only one 

 instance which could be described as a porphyritic diorite. 

 This occurs traversing the gneissic quartz diorite near the 

 contact at Swift's Creek. 



I have prepared and examined slices of various of these 

 dykes, which I now note. 



Dense diorite. — Many dykes of this class occur throughout 

 the district. An example taken from one penetrating the 

 schists on tbe eastern side of the intrusive mass gave me 

 the following : — The rock is micro-crystalline, having a 

 crypto-crystalline ground mass in which are many minute 

 plagioclase prisms, a little iron ore, and a large amount of 

 viridite, to which the green colour of the rock is due. 



Another dyke of this character which penetrates the 

 amphibole granite near the eastern boundary, showed me, 

 when examined as a thin slice, that it consisted of a crypto- 

 crystalline ground mass, having traces of what may have 

 been a glass basis. In this are many long and narrow 

 prisms of plagioclase. A large amount of light-coloured 

 viridite, in flakes and twisted fibres, is distributed throughout 



