Reviews—Dr. Geikie—Tertiary Volcanic Action. 33° 
the Inner Hebrides. The eruptive bosses of basic rocks that have 
broken through the plateaux are next discussed, and then an account 
is given of the protrusions of acid rocks that mark the latest phase 
of eruption in the region. 
Finally, the leading features in the history of Tertiary volcanic 
action in the British Isles are summarized by Dr. Geikie, whose 
words we quote as far as possible in our condensed account of his 
views. 
The earliest beginnings of the volcanic disturbances may possibly 
go back into the Eocene period, and the final manifestations may not 
have ceased until Miocene times, or perhaps later. . 
These disturbances originated from a vast subterranean lake or 
sea of molten rock, which appeared beneath the volcanic region as 
it underwent elevation. Enormous horizontal tension arose, and a 
system of approximately parallel fissures opened in the terrestrial 
crust, having a general direction towards N.W. The majority of 
the fractures did not reach to the surface of the ground, though 
probably not a few did so. No sooner were the fissures formed than 
the molten lava underneath was forced upward into them for many 
hundred or even thousands of feet above the surface of the sub- 
terranean lava-sea. Solidifying between the fissure-walls, the lava 
formed the numerous basic dykes that constitute the widespread and 
distinctive feature of the volcanic region. 
Where the fissures reached the surface or near to it, the molten rock 
sought relief by egress in streams of lava, of which abundant remains 
are still left in the basalt- plateaux of Antrim and the Inner Hebrides. 
In some places, the accumulated pile of such ejections, which 
include layers of fine tuff, etc., even now exceeds 3000 feet. The 
surface over which the lava flowed seemed to have been mainly 
terrestrial, for here and there, between the successive sheets of basalt, 
the remains of land-plants and also of insects have been discovered. 
Subsequently there uprose at certain points, coarsely crystalline 
basic rocks, which solidified as dolerites, gabbros, etc. 
Probably long after the eruption of the gabbros, a renewed out- 
break of subterranean activity gave rise to the protrusion of rocks 
of a markedly acid type. They include varieties that range from 
felsites through porphyries and granophyres into granite. 
Around the bosses of gabbro and granophyre, the bedded basalts 
have undergone considerable contact-metamorphism ; and it is inte- 
resting to learn that the former precisely resemble rocks of similar 
kinds in Paleozoic and Archean formations. 
Ultimately another system of basic dykes was formed; dykes which 
cross those of earlier date, and rise through the other volcanic rocks. 
We have passed over the more detailed portions of this work, 
contenting ourselves with pointing out the leading conclusions of 
the author. These are indeed based on extensive microscopic in- 
vestigations of the rocks, as well as on the more important field- 
work. Further particulars, however, of the microscopic petrography 
are promised in a future memoir on the subject, by Dr. F. H. Hatch, 
whose assistance is cordially acknowledged by the author. 
DECADE III.—VOL. VI.—NO. I. 3 
