498 PROFESSOR GEIKIE ON THE 
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 
themselves here and there into forms like portions of the rhombohedral 
skeletons of titaniferous iron already described. Numerous green serpentinous 
eranules may mark the position of the original augite. There is no trace of 
olivine. 
So thoroughly fused and liquid has the dolerite been at the time of its. 
injection, that little threads of it, less than $5 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. 
With regard to the change superinduced upon stratified rocks in the 
examples now under description, it is not easy to speak precisely, because the 
altered portions are completely enclosed within the mass of dolerite, and we 
cannot tell whence they were derived. They have acquired externally a per- 
cellanite aspect, and show numerous shining granules of quartz. Under the 
microscope this altered rock, which is pale grey or white, has a milky trans- 
lucent finely granular or dusty base, reminding one of the base of a felsite. 
It contains thin layers of quartz pebbles, which are also scattered promiscuously 
through the more compact portions. I have not succeeded in detecting any — 
microlites in this rock, save that here and there, close to its junction with the 
igneous threads and veins, it sometimes contains exceedingly minute cubes and 
irregular grains of a black opaque mineral. The quartz grains contain cavities, 
but are not clear, owing to the presence of many fine fissures and enclosed — 
greenish matter. They may be seen projecting into the dolerite, which has 
moulded itself to them, and has sometimes detached and completely enveloped 
them. 
It is evident that specimens taken from the edge of an intrusive sheet, where — 
the rock has rapidly chilled and solidified, represent to us an earlier stage in 
* The infusibility of the felspar has been well shown in some recent experiments on the rocks of 
the neighbourhood of Edinburgh, At my request, Dr R. S. Marsprn has subjected some of these 
rocks to fusion at the laboratory of the University of Edinburgh, and I have had microscopic sections 
prepared of the products obtained, The basalt of Lion’s Haunch is peculiarly instructive. Its large - 
labradorite crystals 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 definite crystals of 
felspar should have appeared in dykes and veins while the rock was still thoroughly liquid. The 
glass obtained from the Lion’s Haunch rock is of a honey-yellow, and contains translucent tufted — 
microlites. The iron forms beautiful dendritic films in the cracks. Altogether, the glass presents a 
strong resemblance to the peculiar substance found in some of the tuffs of the vents to be afterwards — 
described. I am at present engaged in a series of experiments on the fusion of volcanic rocks and 
artificial slags, and hope to communicate the results in a future paper to the Society. 
iv 
