CARBONIFEROUS VOLCANIC: ROCKS OF THE FIRTH OF FORTH BASIN. 499 
the history of the whole mass than specimens taken from its central portions. 
In fact, a series of samples collected at short intervals from the outer contact 
to the inner mass shows, as it were, the successive stages in the consolidation 
of the molten rock. 
From the observations just described, it appears that the triclinic felspars 
began to assume the shape of large definite crystals before any of the other 
minerals. These felspars already existed when the molten mass forced its way 
among the shales, for they can be seen lying with their long axes parallel to the 
surface of shale, precisely as, in the well known fluid structures, they behave 
round a large crystal imbedded in the heart of a rock. But most of the felspar 
remains still undividualised, together with the other constituents, in a dark 
glassy tachyliticmagma. <A few feet from where the consolidation was not so 
rapid, we perceive that the iron oxides grouped themselves into incipient 
crystalline forms and skeleton crystals ; the felspar crystals formed abundantly, 
though small in size, and the augite was left as a finely granular green trans- 
parent substance. Still further towards the interior of the mass the normal 
character of the dolerite is gradually assumed. 
One of the most constant kinds of alteration is that which affects dolerites 
and basalts where they have been intruded among carbonaceous shales or 
coals. A thin slice of the “‘ white trap” from one of these junctions shows a 
dull white or pale yellowish granular translucent ground-mass, with a few small 
and decayed felspar prisms seattered through it. The absence of the usual 
green colouring matter and of the dark iron oxides is very marked under the 
microscope. The latter minerals seem to have been usually converted into 
siderite and limonite. The augite no doubt existed originally only in the 
minutely granulated form, but it cannot now be distinguished in the general 
kaolinised base. 
III. Tue Basaurs. (Plate XI. figs. 4 and 5.) 
Under this title I include all those rocks of the augite-felspar series which 
have a compact or finely granular base, through which the component crystals 
of triclinic felspar (probably always labradorite) augite, olivine, and magnetite 
are crowded. This base consists partly of a clear or pale-brown glass ground- 
mass, and partly of minutely granulated augite, and microlites probably of the 
felspar. It varies much in amount, sometimes almost disappearing; at other 
times occupying by far the largest bulk of the rock, and with only a few 
scattered crystals of the usual minerals, as in some of the most compact homo- 
geneous basalts. 
The more distinctly crystalline varieties are anamesites. The great majority 
are, however, true basalts. Though there is no essential distinction between 
