TRANSACTIONS OF SECTION C. 133 



mite." Larger crystals are sometimes light purple-brown m 

 the centre and zoned green aromid, rarer occurrences are two 

 purple-brown patches with green between and outside, and 

 still rarer ones are green in centre and greenish-purple around. 

 Maximum extinction observed in the prismatic zone, 42^. 

 Sections cut at right angles or nearly so to the vertical axis 

 show the traces of the prism coP(llO), the orthopinacoid 

 coi^oo(lOO), and the clinopinacoid ooS_ oo(OlO) (see a, Figs. 2 

 and 4), and give a fine optic axis at the edge of the field. 

 Pleochroism pretty strong, especially in the latter sections, 

 varying between deep grass-green (b) and greenish-yellow (c). 

 Amongst the larger crystals, some are zoned quite black 

 around the sides with fine black dust of iron-ore ; others con- 

 tain a number of larger grains of the ore ; and there occur 

 cases where the whole crystal is densely filled with such 

 grains throughout. Twins seem rather rare, as only a few 

 (with coPcxD (100) as twinning-plane) were observed iu a number 

 of sections. 



Hornblende. — This is in the Portobello rock quite as 

 abundant as, if not more so than, the augite. In the Pine Hill 

 rock it is less frequent. Its colour is light-brown, and it forms 

 slender columnar crystals, varying iu size from 3m-4m. long 

 and l-2mm. broad (/i. Fig. 4) to very small dimensions, less 

 than Q-Oomm. in length. Maximum extinction observed, 10°. 

 Pleochroism very strong, varying between light yellow'ish- 

 brown (o) and deep rust-brown (c) ; absorption c > b > a. 

 Sections at right angles to the vertical axis are of hexagonal 

 outline, formed of the traces of the prism ooP (110), and of the 

 clinopinacoid, oo-P- c« (010). (See Ji, Fig. 5.) Twins occur 

 rarely. Like those of augite, the crystals of this mineral are 

 also, though apparently less strongly, invaded by dust and 

 grains of iron-ore ; whilst some show but small specks of their 

 brown colour, and others appear quite opaque-black through 

 this invasion, there occur again large crystals []i, Fig. 4) which 

 are quite clear throughout — a feature not observed in large 

 crystals of augite. 



Olivine. — In the Portobello rock inclusions of this mineral 

 are rather abundant and pretty evenly distributed throughout 

 the dark base, as nearly every thin section shows several of 

 them. This is not the case in the Pine Hill rock, as some 

 sections of this contain not one inclusion, whilst in others 

 there is quite a number. The mineral is transparent and 

 colourless, sometimes with a slight tinge of greenish-yellowy 

 and it occurs in irregularly-outlined grains and more or less 

 perfect crystals, which both go rarely down to less than 0-5mm. 

 in diameter, but generally vary between l-3mm. in size. They 

 are usually enclosed by a deep-brown or opaque-black ferru- 

 ginous rim of greater or less width, which probably presents a 



