346 Proceedings of Royal Society of Edinburgh, [may 21, 
these fissures basic lava rose from a subterranean sea of molten rock 
some 40,000 square miles in extent, which stretched under the 
north and west of the British Isles, and thus formed the first series 
of basic dykes. Though most of the fissures doubtless terminated 
before reaching the surface, not a few of them probably extended 
upward to it, and in these cases a communication was opened be- 
tween the heated interior and the outer atmosphere. It was from 
vents formed in these fissures that the great lava streams of Tertiary 
time proceeded. 
II. The second part deals with the volcanic phenomena thus 
established. First, an account is given of the petrography of the 
different materials ejected to the surface. The lavas are varieties of 
the great basalt famil}', but include some curious pale species with a 
specific gravity of 2 -71 to 2'74, and containing little else than felspar. 
The fragmental rocks comprise coarse volcanic agglomerates, also 
conglomerates and breccias, some of which contain large blocks 
of quartzite, schist, and other non-volcanic materials. There are 
likewise tuffs, fine clays, limestone, gravel, leaf-beds, and lignite, 
the vegetation preserved in these intercalated strata being terrestrial, 
and undoubtedly of older Tertiary age. The author then proceeds 
to describe the structure of each of the basaltic plateaux of Britain, 
— those of Antrim, Mull, Small Isles, and Skye. He points out 
the absence of any proof of great central vents, and shows from the 
general horizontality of the basalts, their thinning away in different 
directions, and the thinness, local development, and want of per- 
sistence of their associated tuffs, that the volcanic vents must have 
been numerous and of comparatively small size. Some of these 
vents are still to be seen, filled up with dolerite or with agglomerate. 
The volcanic plateaux of Britain find their exact counterparts in the 
younger lava-fields of Western America. 
III. The third part treats of the great eruptive bosses and sheets of 
gabbro, dolerite, &c., which have broken through the basalt plateaux. 
After describing the petrography of these rocks, the author gives an 
account of their modes of occurrence and their relation to the other 
volcanic rocks in the four plateau districts. He shows that in each 
case there is an amorphous core of comparatively coarse granitoid 
gabbro, or dolerite, from which proceed intrusive sheets or sills into 
the surrounding basalt plateaux. The bedded basalts pass below. 
