170 



PEOr. T. T. GKOOM ON THE IGNEOUS ROCKS [Feb. I9OI, 



In addition to the andesine felspar-laths, simple or twinned, and 

 not uncommonly reddish along the borders, are small idiomorphic 

 crystals and grains of a pale angite, with an extinction-angle of 

 about 43° and good prismatic cleavage. These are often very abun- 

 dant. The microlites are sometimes elongated, and lie parallel to the 

 felspars. There is much scattered ilmenite and titaniferous 

 magnetite, both in the form of irregular grains and small crystals. 

 Long needles, or short prisms of apatite, are often enormously 

 abundant, especially in the felspar ; these are best seen in wea- 

 thered examples (PI. YII, fig, 4). There are also many scattered 

 patches of clear isotropic serpentine, partly, and I believe largely, 

 after augite. These patches in the more altered rocks are often 

 impregnated with finely-divided calcite. 



Some of the felspar-laths in the immediate yicinity of the 

 serpentine-phenocrysts are almost invariably impregnated with 

 serpentine, and in some of the more altered rocks, such as M 236, 



Fig. S.—OUvine-hasalt (M 118). 



Portion of the groundmass showing fibrous felspar-laths, iron-ores, augite- 

 miorolites (with refriugent borders), and patches of serpentine (light in the 

 figure). [From a photograph.] 



the majority of the laths have been so affected, and give the impres- 

 sion of pseudomorphs after a ferromagnesian mineral. 



In all the oli vine-basalts there is a fair quantity of biotite, the 

 fresh appearance of which at first sight seems to mark it out as an 

 original constituent. In the fresher rocks, however, such as M 214, 

 the mica is largely confined to the serpentine, and the size of the 

 flakes is to a certain extent proportional to that of the patch of ser- 

 pentine in which they lie ; this mode of occurrence points definitely 

 enough to the secondary, or rather to the tertiary, origin of the mica. 

 The production of biotite has taken place most frequently in the 



