142 REPORT— 1891. 



arranged. Some amongst the sections of the mineral show 

 marginal growth, i.e., a narrow rim, optically of different 

 orientation from the enclosed portion. Large colunmar crys- 

 tals of plagioclase are also present, though in less number 

 than those of sanidine. Both feldspars occur occasionally grown 

 parallel togethei' ; and there are sections showing a parallel 

 intergrowth of plagioclase and sanidine : i.e., a large centre 

 portion of the former is surrounded by a narrow zone of the 

 latter, exactly as shown in fig. 1, plate xxiv., H. Eosenbusch's 

 " Physiography of Eock-making Minerals" (translated by J. P. 

 Iddings). Green augite is plentiful in large and small crys- 

 tals, which are generally impregnated with black grains and 

 dust of iron-ore in greater or less abundance. Some of the 

 larger crystals are brown in the centre and green around. 

 Hornblende occurs very sparingly in good-sized crystals, whicli 

 are for the greater part black-opaque, only in places deep- 

 brown-translucent. Apatite is tolerably frequent. Iron-ore 

 in large and small grains abundant. 



12. Occurs at the Bottom of the Cliff in the White Decompo&cd 

 BocJc hetioeen the Dykes of Coarse Porphyry, Portobello 

 Promontory. (A similar rock croi^s out on top of the 

 j^romontory.) 



Bock. — Greenish-grey, fine-granular, and porphyritic. Con- 

 tains scattered large patches of black hornblende. Gives a 

 fair amount of gelatinous silica on treatment with HCl. 



Sections. — As in macroscopic aspect, so also in microscopic 

 character, there exists a marked resemblance between this 

 rock and the one just described (No. 11), though the localities 

 are nearly ten miles distant from each other. It is an analo- 

 gous case to that of the coarse porphyries of the Portobello 

 promontory and Pine Hill. The ground-mass and microlites of 

 these slides and of No. 11 are quite undistinguishable, and, as 

 regards the porphyritic inclusions, the only differences are that 

 olivine — absent in No. 11 — occurs in these slides sparingly in 

 large grains, partially or wholly serpentinised and surrounded 

 by broad black margins ; also that the sanidines and plagio- 

 clases are larger and more abundantly filled with inclusions 

 (ground-mass, iron-ore, &c.), some showing only a very narrow, 

 clear, marginal zone. Of marginal growth of sanidine and of 

 parallel intergrowth of sanidine and plagioclase, as noticed in 

 No. 11, there occur several fine examples. One large crystal 

 of sanidine shows two narrow marginal zones of different optical 

 orientation, surrounding a differently - orientated centre part. 

 Another large inclusion exhibits a columnar plagioclase sur- 

 rounded by two differently-orientated broad parallel zones of 

 sanidine. 



