CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 517 
EXPLANATION OF PLATES. 
Piate IX. Map of the Volcanic Districts in the Basin of the Firth of Forth. 
Pirate X, Vertical Section of the Lower Part of the Carboniferous System in the Basin of the 
Firth of Forth, showing the succession of Volcanic Eruptions. 
PiatE XI. Microscopic Structure of the Volcanic Rocks of the Basin of the Firth of Forth. 
Fig. 1. Diabase, Crossall Hill, Linlithgowshire——The felspar is chiefly orthoclase in Carls- 
bad twins, with the “herring-bone structure” described on p. 488. The augite occurs 
in large crystals and aggregations of crystals of a delicate claret colour. Some large 
crystals and a portion of a remarkable compound crystal of titaniferous iron are 
shown. The long white rod and the numerous colourless hexagonal sections are apatite. 
A few brown fibrous plates of biotite appear. (20 diameters.) See p. 487 e¢ seq. 
Fig. 2. Diabase, Corstorphine Hill, Edinburgh.—The augite is conspicuous in the centre of 
the field, surrounded by a turbid milky felspar. The opaque titaniferous iron appears 
to shade off into a whitish dull translucent substance (leucoxene). Patches of 
bright green decomposition products, sometimes with tufted fibrous structure, fill up 
some of the interstices. (20 diameters.) See p. 487 e7 seq. 
lo 
im 
2 
e. 3. Dolerite, Dalmahoy Hill, Edinburgh.—An intrusive rock, showing abundant large 
clear prisms of a triclinic felspar, numerous large but rather ill-defined crystals of 
pale brownish pink augite, which has sometimes enclosed the felspar prisms. The 
titaniferous iron occurs in smaller forms than in the coarser diabases. Between the 
various minerals a considerable proportion of a ground-mass is interposed, which is 
in large measure devitrified by the appearance of microlites, and which now encloses a 
good deal of green decomposition-products in tufts, threads, and streaks. It is likewise 
traversed by clear needles of apatite, and marked by brown spots of limonitic dis- 
coloration. (20 diameters.) See p. 493. 
Fig. 4. Anamesite, Craiglockhart Hill, Edinburgh.—This section is placed beside fig. 3 
to show the distinction between intrusive and bedded rocks of the dolerite type. It 
shows a crystalline admixture of clear labradorite prisms, with abundant granular augite, 
through which are scattered a few large, well-formed crystals of the latter mineral, 
with crystals of olivine, usually serpentinised. One large well-defined olivine, with 
its green transverse decomposed portions and the central still comparatively fresh 
kernels, forms a prominent feature in the drawing. The iron is in very minute forms, 
and appears to be chiefly magnetite. (20 diameters.) See p. 501 et seg. 
Fig. 5. Basalt, Kirkton East Quarry, Bathgate——This section represents the structure of a 
typical interbedded basalt of the district. The rock evidently consists of an intimate 
mixture of minute prisms of labradorite and granular augite, between which clear 
interstices appear filled with a glassy ground-mass. A few well-defined usually 
compound crystals of augite are interspersed, but are not so abundant or conspicuous 
as the olivines, which are almost invariably converted into green serpentine. Octo- 
hedra of magnetite are tolerably uniformly dispersed through the rock. The minutely 
granular condition of the augite in this rock and in fig. 4 may be contrasted with that 
of the intrusive rocks Nos. 1, 2, and 3. (20 diameters.) See p. 501 et seq. 
VOL. XXIX, PART I. 6k 
