400 DR ROBERT CAMPBELL ON 



olivine, plagioclase, ilmenite, and brownish-green augite. • Olivine is in much greater 

 abundance than in the other trachydolerites, and occurs in a second generation of 

 small crystals in the groundmass. The olivines are sometimes quite fresh, some- 

 times completely replaced by pale brownish-red " iddingsite." Ilmenite is found in 

 large plates and skeletal growths, in part enclosed in the porphyritic olivines, in part 

 moulded in the felspars. The plagioclase phenocrysts are not numerous, and are 

 slightly more basic than in the other varieties. Extinction angles determined in 

 sections cut at right angles to 010, and showing Carlsbad and albite twinning, prove 

 that the felspar is, in part at least, a labradorite-bytownite with the composition 

 Ab 3 An 7 . Not infrequently the crystals are surrounded by a narrow zone more 

 acid in character and crowded with inclusions. 



The groundmass consists chiefly of stout laths of labradorite, thin laths of 

 oligoclase-andesine, olivine, and granular augite, along with accessory iron oxides 

 and apatite. The interstitial alkali felspar is in much smaller amount, and the 

 groundmass is more coarsely crystalline than in the acid variety. 



The two remaining specimens of trachydolerite [G. 6 and G. 7] differ from the 

 types already described in being conspicuously porphyritic with glassy plagioclase 

 (labradorite-bytownite), augite, and olivine (PL XXXVI. fig. 5). They contain also 

 microporphyritic crystals of apatite and ilmenite. G. 7 is very rich in brown dichroic 

 apatite, similar to the brown apatite of the trachytes ; in G. 6 the apatites are also 

 numerous, but are colourless. Glomeroporphyritic aggregates of olivine, apatite, and 

 ilmenite are not uncommon. 



Their groundmass is intermediate in character between those of the acid and 

 basic types. It consists for the most part of laths of oligoclase and oligoclase- 

 andesine, granules of greenish non-dichroic augite, iron oxides, and thin needles of 

 apatite, together with a matrix of anhedral albite and albite-oligoclase. A small 

 amount of residual glass is present in G. 6. 



The trachvdolerites agree in all possessing in varying degree a considerable 

 development of a crystalline "cement" of alkali or alkali-rich felspar, which is 

 untwinned, and has a mean refractive index well below that of the Canada Balsam of 

 the slides. In order to ascertain to which variety of felspar the " cement" belonged, 

 it was separated from the other constituents in the powdered rocks, and its refractive 

 indices compared with those of oils of known refraction. It was found that in all 

 the specimens it consisted of albite and albite-oligoclase. Examination of several 

 fragments in convergent light revealed their optically positive character. None 

 of the felspar observed had refractive indices so low as those of orthoclase and 

 anorthoclase. 



Glassy basalts (? trachydolerites). 



A rounded stream boulder of a vesicular lava [G. 15] contains numerous pheno- 

 crysts of olivine, titanaugite, and basic plagioclase in a groundmass of brown glass 



