502 PROFESSOR GEIKIE ON THE 
grains and crystals, and is generally completely altered into a yellowish-green ser- 
pentinous substance. This is the ordinary mode of occurrence of the mineral. 
The magnetite occurs in octohedra, averaging perhaps ‘001 inch in diameter, 
also in many larger and irregular aggregations. Some of the latter, however, 
may be titaniferous iron. Apatite appears rarely in slender needles. 
Between these recognisable minerals there lies a minutely granular base, 
which can be resolved by a high power chiefly into augite, with here and there 
traces of a dusty devitrified substance. But it cannot be called a glassy ground- 
mass. The proportion in which it occurs to the rest of the constituents is so small 
that the crystalline granular character of the rock remains conspicuous. 
(2.) The true basalts vary greatly in details, but agree in the following 
general characters. The triclinic felspar occurs in minute prisms (perhaps on 
an average ‘0005 of an inch thick and ‘005 of an inch long) seldom in large © 
porphyritic crystals. The augite appears in its two forms, but while the 
granulated condition is always present, the larger definitely shaped crystals are — 
often absent. The olivine, always in serpentinous pseudomorphs retaining the 
rude contour of the original mineral, frequently lies in large porphyritic 
crystals. Magnetite forms a conspicuous feature, though its proportional amount 
is subject to great variations. The structure of the rock is always minutely 
granular, the granules consisting of augite; but between them, and in the 
wedge-shaped angles between the felspar prisms, a clear glass with pellucid — 
spicules may sometimes be observed. (See fig. 5, Plate XI.) 
The felspar is on the whole the predominant mineral, but this chief part is 
not infrequently taken by the granular augite. As in the dolerites, the felspar 
encloses globules of augite often less than ‘0001 of an inch in diameter, also 
specks of magnetite. In fresh specimens it remains exceedingly clear and quite 
unchanged. It is always well striated. In some rocks its minute prisms are — 
arranged in the most perfect fluid structure along the faces of the large 
olivines and augites. A good example of this arrangement is supplied by a — 
very compact basalt on the shore to the west of Pettycur. | 
The augite granules average perhaps about ‘001 of an inch in diameter. 
On applying a high power to their examination they are often seen to have 
imperfect crystalline outlines. That they are crystalline bodies, and not mere 
vlass, is shown by their behaviour under polarised light. In some basalts— 
those of Kirkton, near Bathgate, for example—they appear as it were curdled — 
found in rings, the external outlines of which are rudely those of augite — 
prisms, while the interior is occupied by a confused mass of augite granules, — 
magnetite, and the general ground-mass of the rock. The colour of the — 
