268 Transactions. 



In hand-specimen the camptonites are fine-grained blackish rocks show- 

 ing few, if any, determinable crystals. Under the. microscope the most 

 noteworthy features are the precise idiomorphism of the constituent mine- 

 rals, the great abundance of augite (which forms nearly 50 per cent, of the 

 rock), and the lack of importance of feldspar. The essential minerals 

 are pseudomorphs of iddingsite after olivine, violet titaniferous pleochroic 

 augite, brown hornblende, magnetite, and plagioclase. The only porphy- 

 ritic mineral is augite, which, with olivine and occasional iron-ore, forms 

 the phenocrysts of the porphyritic types ; even in the groundmass, where 

 its stumpy prisms exclude almost all other minerals, it is sharply idio- 

 morphic. In the porphyritic types the pyroxene is usually in " glomero- 

 porphyritic " phenocrysts, the individual crystals of which show good 

 " hour-glass " structure. 



Olivine is represented solely by pseudomorphs of yellowish -green iddings- 

 ite ; these are characteristically idiomorphic, moderately coarse, and very 

 plentiful. In one instance a little serpentine showing " mesh " structure is 

 associated with the iddingsite of a pseudomorph. In the rock from one 

 dyke olivine is absent, unless it is represented by rounded pseudomorphs 

 of carbonate, which are particularly abundant, and in some cases possess 

 an outline characteristic of olivine. The amphibole is a deep-brown, rather 

 poorly pleochroic variety, probably barkevicite ; it forms characteristic 

 slender cigar-shaped prisms, which, with a moderate amount of fine mag- 

 netite, crowd the clear feldspathic base between the numerous stumpy 

 augite crystals of the groundmass. The feldspar — andesine-labradorite — • 

 is in long irregular laths usually of small size, or in small crystals embrac- 

 ing all the other minerals and crowded with numerous colourless acicular 

 crystallites. Occasionally it forms most curious arborescent growths. 

 Secondary carbonates are usually abundant. 



In some sections — namely, those from the dyke in Haggard Creek — the 

 iron-ore is in coarse well-shaped crystals typical of magnetite ; zonal inter- 

 growth with ilmenite has taken place, and is represented by a more or less 

 regular internal band of leucoxene parallel to the periphery of the crystal. 

 In the clear portions of the groundmass microscopic pinnate growths of 

 iron-ore are often present. 



In a section made from one of the Blackwater River dykes several clear 

 glassy rounded patches of isotropic mineral were observed ; these probably 

 represent a feldspathoid mineral, but the rock was too decomposed to 

 permit of determinative tests being applied. 



Monchiqniles. — Boulders of typical monchiquite were found by Mr. 

 P. G. Morgan in Rider Creek, a branch of Haggard Creek ; and, in addition, 

 as already mentioned, sections from the camptonite dyke in Haggard Creek 

 have a glassy base, and can well be classed as monchiquite. 



There is a general similarity in mineral constitution between the mon- 

 chiquite and the porphyritic camptonite already described. Olivine, much 

 of which is fresh and the rest pilitized (talc and tremolite), is in large 

 remarkably idiomorphic phenocrysts, which are often surrounded cely- 

 phitically by augite. This latter mineral differs from that in the camp- 

 tonite in that brown hornblende very frequently forms a parallel terminal 

 outgrowth to its prisms, but otherwise shows the same features. In the 

 groundmass it is in segregations, along with a little iron-ore. These are 

 enwrapped by a clear base containing plentiful laths of feldspar and crowded 

 with crystallites, some of which appear to be green hornblende. Staining 

 tests reveal the presence of a few minute needles of feldspar, and indicate 

 that a moderate amount of gelatinizing isotropic material is also present. 



