88 Transactions. 



Dykes. 



|The granitic rocks are penetrated here, as elsewhere on the Coast, by 

 numerous basic and semibasic dykes, inckiding varieties ■ of diorite, por- 

 phyrite, camptonite, and similar types. 



The slates are also intruded by a series of dykes which, so far as yet 

 known, are exclusively of diabase. The outcrops are difficult to locate, 

 owing to the extensive alteration which the dykes have undergone, some 

 of them being slightly schistose. They have generally been exposed in 

 mine-workings only, and, as many of the mines are now closed down, 

 observations cannot readily be made. They have been found in the 

 Specimen Hill, Inglewood, and Keep It Dark Mines, but the only one 

 that I saw was in the last-named, and its occurrence is described under 

 the description of that mine. 



It is notable that these dykes seem confined to the gold-bearing belt, 

 but their intrusion seems to antedate the vein-formation. Their intense 

 alteration is probably due to solutions acting at this period, and their 

 occasional schistosity indicates that they have been subjected to considerable 

 pressure, probably during the period of folding of the slates. There is 

 thus evidence for assuming that they are the oldest known intrusives in 

 the district. 



Petrography of Diabase. 



Megascopically the diabase is a rather soft dark-green rock, full of black 

 cleavage-faces of small augite crystals. 



Under the microscope the rock, when fresh, has the typical ophitic 

 structure of a diabase. It is composed of twinned laths of andesine-feldspar 

 containing some secondary sericite and calcite arranged evenly along the 

 centre of the crystals. Augite is fairly fresh and pale in colour, the crystals 

 filling up interspaces between the feldspars, or occurring as small detached 

 crystals. The individuals are occasionally twinned. The relative propor- 

 tions of augite and feldspar vary somewhat ; at times augite predomi- 

 nates, with the local disappearance of other constituents. It is then 

 coarse and in better-outlined crystals. Olivine is occasionally seen in some 

 quantity, but very largely replaced by a mesh of serpentine, without the 

 separation of much iron-oxide. It is probably not a highly ferriferous 

 variety. Of other constituents, a good deal of magnetite (both primary 

 and secondary), a little basaltic hornblende (often chloritized), and needles 

 of apatite are present. 



The alteration of this rock close to the vein is well shown under the 

 microscope. In the first stage the feldspars become more and more replaced 

 by carbonates till the twinning is indistinguishable, and they practically 

 consist of sericite-calcite pseudomorphs. Olivine is replaced by chlorite 

 and serpentine, augite by magnetite (or ilmenite) and a leek-green serpen- 

 tine. The ophitic structure can, however, still be traced. 



With further alteration all structure is lost, even the outlines of crystals 

 being obliterated. The rock becomes an aggregate of carbonates and 

 sericite, with a good deal of opaque iron-oxide, and a little quartz, probably 

 introduced. Strings and patches of pale-green chlorite are present at first, 

 but these are eventually altered to sericitic matter. Pyrite crystals are also 

 frequently present. 



Thus we are able to trace the progressive steps of alteration. The lime- 

 soda feldspar is changed to carbonates and sericite; the augite and olivine 

 first to transitional chlorite and serpentine, with separation of iron-oxides, 



