168 



Whin Sill were strongly magnetic and at the same time contained 24 p.c. 

 of titanic oxide. 



Iron oxides are generally regarded as constantly occurring in the basic 

 eruptive rocks as original constituents. Professor JUDD, however, holds that 

 this is not the case in the gabbros of the West of Scotland. In these rocks 

 the iron oxides do not appear to have separated out during the consolidation 

 of the magma ; they occur in cracks in the olivine and are regarded by him 

 as the result of the " schillerizing " process. In rocks of the dolerite and 

 basalt families, and their vitreous representatives, opaque iron oxides are 

 constantly found. Their separation from the magma does not appear to be 

 limited to any particular phase in the consolidation. In the metamorphosis 

 of dolerite into hornblende schist the decomposition of the original titaniferous 

 magnetite or ilmenite gives rise to the formation of small vividly polarising 

 granules of sphene. 



Black Mica. This mineral occurs in flat hexagonal tables. It possesses 

 a perfect cleavage parallel to the basal plane. Sections at right angles to this 

 plane show the cleavage cracks running parallel with the edges. They are 

 lath-shaped in form and give straight extinction. They are also strongly 

 dichroic ; changing from a pale brown, when the short axis of the nicol lies 

 at right angles to the length of the section to a deep brown, or almost black, 

 when the short axis lies parallel with the length of the section (distinction 

 from brown tourmaline). Sections parallel with the basal plane remain dark 

 in all positions between crossed nicols, are not dichroic and give the interference 

 figure of a biaxial crystal with small axial angle. 



The minerals which remain to be noticed occur only in very small 

 quantity or as occasional constituents of certain rocks. Quartz, as might 

 naturally be expected, is very rare in the basic eruptive rocks as an original 

 constituent. It is found in some of the more acid gabbros and dolerites 

 (diabase), and is then frequently associated with felspar as a constituent of 

 micro-pegmatite. It arises also as a secondary product in consequence of the 

 alteration of the original constituents by surface agencies, and in this 

 condition is not uncommon. Apatite is almost invariably found. It occurs 

 as short stumpy prisms and also as acicular microlites. Cross-sections are 

 hexagonal and the angles are usually sharp and well defined. It is generally 

 colourless ; but occasionally shows grey, blue or yellowish tints. The 

 refraction is high. The double-refraction is slight and of a negative 

 character. It is readily acted upon by acids and the solution gives with 

 molybdate of ammonia the Avell known yellow precipitate. This test may be 

 easily applied to microscopic sections and furnishes a ready method of 

 distinguishing this mineral from nepheline. Apatite varies considerably 

 in amount in different rocks, and in a few cases is extraordinarily abundant. 

 It is always one of the earliest products of consolidation, and occurs therefore 

 as inclusions in all the other minerals. 



Zircon is now known to be a widely distributed mineral in all kinds of 

 igneous rocks. (1) It is, however, more common in the acid than in the basic 



(1) THURACK. Uher das Vorkotmtieii mikro.skopischer Zirkone und Titanmineralien 

 in den Gesteineii. Verhdl. d phys.-medicin Ges. Wiirzburg, 1884. 



