CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 493 
Tertiary age. They consist of undoubted and typical dolerite. They run 
across some of the great igneous sheets in the Coal-measures, and can there be 
admirably compared with the older rock. After examining them over a wide 
area in the field, and having had many slices cut from both series of rocks for 
microscopic investigation, I could never distinguish between them except by 
reference to their respective labels. Accordingly, I gave up this term green- 
stone in favour of dolerite, and this change has been adopted in all the 
subsequent publications of the Geological Survey in Scotland. 
My friend, Professor Z1IRKEL, who visited me in Ayrshire, and went over 
some of the evidence on the ground, came to the conclusion that no difference 
was to be made out by the microscope between the Carboniferous igneous rocks 
and those of Tertiary date.* Mr Aiport has come to the same conclusion.t 
But while no recognisable distinction can be drawn between Carboniferous 
and Tertiary dolerites, I have been led to discover that a definite line of 
demarcation can be drawn between the intrusive dolerites and the augitic lavas 
which have been erupted at the surface. On a former page (ante, p. 481) I have 
referred to some of the broad external features of difference. But the micro- 
scope helps still further to discriminate them, and furnishes a valuable assistance 
in this respect to the labours of the geologist in the field. So reliable indeed 
are the microscopic tests, that I believe it is possible, in most cases at least, to 
affirm, even from the small portion of rock placed under the microscope, 
whether the parent mass consolidated beneath ground or at the surface. 
Chemically there is probably, as a rule, little or no difference between the 
intrusive and interbedded sheets. Their differences lie in structure and 
texture, and point to the opposite conditions under which the rocks acquired 
solidity. While the intrusive sheets are conspicuously crystalline dolerites, the 
interbedded are essentially basalts of varying degrees of compactness. I 
would, therefore, restrict the term dolerite to the one, and basalt to the other 
petrographical group. 
1. General External Characters and Mode of Occurrence.—In fresh 
undecayed specimens the texture of dolerite is markedly crystalline, passing 
into crystalline-granular. The component minerals can usually be distinguished 
either with the naked eye or a lens, the triclinic felspar appearing in clear 
glassy finely-striated prisms in the dark green or black base of augite and 
titaniferous iron. Hence the rock commonly presents a dark-grey speckled 
appearance. Where alteration has made progress, brown tints prevail on the 
outer crust, and the rock crumbles into a brown or even yellow sand, as its iron 
is converted into limonite. 
An intrusive sheet is hardly ever amygdaloidal; when amygdules do 
* “ Mikroskopische Beschaffenheit der Mineralien und Gesteine,” 1873, p. 291. 
t “Quart. Journ. Geol. Soc.” wt supra. 
VOL. XXIX PART I. 61 
