488 PROFESSOR GEIKIE ON THE 
magma, which is no longer recognisable. The chief component minerals are 
orthoclase, plagioclase, augite, titaniferous iron, and apatite. The general 
aspect of these rocks is represented in figs. 1 and 2, Plate XI. 
The felspars usually constitute the main mass of the rock; but sometimes 
the augite is nearly equally abundant. Next in amount comes the iron. The 
proportion of the apatite varies within wide limits, being sometimes limited to 
infrequent stout prisms, at other times diffused in abundant minute needles. 
The orthoclase, in the more typical examples, occurs to the almost total 
exclusion of any triclinic felspar. It is always somewhat kaolinised, but fresh 
portions may often be observed. It occurs macled in the Carlsbad form, and — 
often shows its characteristic divergent herring-bone lineation from the plane 
of twinning. The alteration has resulted in the production of a finely granular 
substance, which between crossed Nicols appears dusted over with bright and 
coloured points, representing, doubtless, the silicic acid of the original felspar. 
The orthoclase prisms are frequently crowded with minute needles and larger 
hexagonal prisms of apatite, either promiscuously through the whole felspar, or 
directed from the exterior towards the centre, which may remain comparatively 
clear. In the more decomposed rocks the orthoclase has acquired a yellowish 
hue from diffused limonite. 
The triclinic felspar, in the coarser and more typical varieties, is prokal 
never labradorite. It differs from the characteristic labradorite of the dolerites — 
and basalts in being less finely striated, and much more liable to decomposition, _ 
It often presents a milky appearance, which under a high power is resolved 
into a fine kaolinised substance. So generally is it decomposed, that though 
the external form of its prisms may remain distinctly marked off from the 
surrounding ingredients of the rock, no unaltered portions may remain, or if they 
occur they lie in small insular spaces amidst the surrounding granular kaolin. In — 
proportion as the orthoclase diminishes, the triclinic felspar appears to increase. 
In the less largely crystalline varieties, however, it ceases to present itself as the — 
milky, kaolinised albite-like mineral, but takes the usual water-clear well — 
striated form of the labradorite, so characteristic of the typical dolerites and — 
basalts of the region. 
Augite is certainly the most conspicuous mineral under the microscope. — 
This arises not only from its abundance in most of the rocks, but from its 
frequently and remarkably undecomposed aspect. Its freshness even in the 
presence of zeolites, “ viridite,” and other proofs of considerable alteration, is — 
not a little singular. It is found in definite crystals of the usual eight-sided 
forms met with among igneous rocks ; though the prisms are usually imperfect _ 
and are sometimes twinned, good cleavage angles of 876’ are common. The 
augite may sometimes be observed enclosing prisms of the milky or finely 
granular triclinic felspars, which shoot through it and are wrapped round by it, 
