474 DR. A. J0WETT OK THE [Oct. I9I3, 



A peculiar type of felspar giving rhomboidal sections is found 

 sporadically in many of the olivine-basalts. The crystals are 

 rarely twinned. 



Olivine. — No fresh olivine has been found in any of these 

 rocks. The commonest type of pseudomorph has a green ser- 

 pentinous interior bordered by reddish-brown material, which also 

 traverses the irregular cracks in the crystal. This type passes 

 gradually into a type entirely serpentinous on the one hand, and 

 into a type devoid of green decomposition-products on the other. 

 Crystals of the latter type are sometimes pleochroic and fibrous, 

 resembling iddingsite. The green alteration -product is fre- 

 quently fibrous, pleochroic, and strongly birefringent. In the more 

 altered rocks it becomes colourless, but still exhibits strong double 

 refraction. When most altered, the olivine is represented by 

 pseudomorphs in rhombohedral carbonates and iron-oxides. 



In many of the rocks in which phenocrysts of olivine are present, 

 small crystals also occur in the ground-mass. In some types, the 

 olivine, although one of the first minerals to crystallize, never 

 attains a greater size than that of the other minerals of the 

 ground-mass. 



Monoclinic pyroxene. — The augite is pale green to 

 ■colourless in thin section. The larger crystals are commonly 

 twinned, but the mineral is most abundant in the ground-mass in 

 a granular form, though occurring (rarely) as small prisms. 



Rhombohedral carbonates are the usual decomposition-products, 

 though sometimes the augite is replaced by chloritic material. 



Rhombic pyroxene. — A very fresh, non-pleochroic, rhombic 

 pyroxene occurs in some of the olivine-basalts near Montrose. It 

 is sometimes intergrown with augite; but, more generally, each 

 crystal of enstatite has a border of granular augite. 



Green faintly -pleochroic bastite is the most characteristic 

 alteration-product, but further alteration produces rhombohedral 

 carbonates and even secondary quartz. In the much-altered rocks 

 it is not always possible to distinguish satisfactorily between the 

 rhombic and the monoclinic pyroxenes. 



Rhombic pyroxene has not been found as a constituent of the 

 ground-mass of any of the rocks. 



A glomeroporphyritic aggregate of bastite, enclosing olivine and 

 basic plagioclase, was found in a section of one of the lavas west of 

 Boddin Point. 



Magnetite occurs as well-formed octahedra in the ground-mass 

 of some of the rocks. In the granular form, it is always abundant 

 in the interstitial glass. Haematite and limonite are also 

 abundant. 



Apatite-needles occur in all the rocks. 



Biotite is fairly abundant in a few rocks. It occurs as clear 

 crystals, one giving the characteristic optical figure, and as opaque 

 rods and fibres. 



