454 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
only iiiiiieral distinctly formed into definite crystals. It occurs in prisms of an 
eail^y consolidation, sometimes one-fifth of an inch long, and therefore readily 
recognizable by tire naked eye. These prisms are perfecth' shaped, contain 
ahnndant twin lamelhe, and show enclosures of the iron of tire base. They 
had been already completely formed at the time of injection; for occasion- 
ally they may he observed projecting beyond tlie wall of the vein into the 
adjacent shale or sandstone, and they have ranged themselves parallel to the 
sides of the vein.^ The black ground, from which these large well-defined 
crystals stand out prominently, consists of a devitrified glass, rendered dark 
by the multitude of its enclosed black opaque microlites. These are very 
minute grains and rudely feathered rods, with a tendency to group them- 
selves here and there into forms like portions of the rhombohedral skeletons 
of titamferous iron. So thoroughly fused and liquid has the dolerite been at 
the time of its injection, that little threads of it, less than of an inch in 
diameter, consisting of the same dark base, with well-defined felspars, may 
be seen isolated within the surrounding sedimentary rock. Minute grains 
and rounded portions of the latter may also be noticed in the marginal parts 
of the dolerite. 
It is thus evident that specimens taken from the edge of an intrusive 
sheet, where the rock has rapidly cliilled and solidified, represent to us an 
earlier stage in the history ot the whole mass than specimens taken from 
its central portions. In fact, a aeries 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 ol )servations 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 lyiiTg with their long 
axes parallel to the surface ol shale, precisely as, in the weil-knowm flow- 
structures, they liehave round a large crystal embedded in the heart of a 
rock. A few feet from whore the consolidation was not so rapid, the iron 
oxides have grouped themselves into incipient crystalline forms and skeleton 
crystals ; the felspar crystals abundantly occur, and the augite has been left 
in the finely granular condition. Still further towards the interior of the 
mass, the normal character of the dolerite is gradually assumed." 
Tlio infnsibility of tlie felspar was well shown iu some experiments on the rocks of the 
neighbourhopil of Eilinbnrgh, made at my roqnest by Dr. R. S. Mar.sden, who subjected some of 
lesc rooks to fu.sion at the laboratory of the Uuivcr.sity of Edinburgh. Microscopic sections were 
pieparoJ ot the products obtained. The basalt of Lion's Hannch i.s peculiarly instinctive. Its 
laigc labriidorito cry.stals have resisted the intense white heat which, continued for four hours, has 
reduced the rest of the minerals to a perfect glass. We can thus well understand how large 
c ennite crystals of felspar should have survived or ajipeared in dykes and veins while the rock 
wa,s still thorougldy liquid. The glass obtained from the Lion’s Haunch rock is of a honey- 
ye low, and contains translucent tufted microlites. The iron form.s beautiful dendritic films in 
the cracks. Altogether, the glass presents a strong resemblance to the palagonitic substance so 
abundant among the lapilli in the tuffs of the vents. 
detailed investigation of the contact phenomena of the Carboniferous 
doleritic sills of the basin of the Firth of Forth, see the papers by Dr. Steelier, quoted 
on p. 451. 
