1888.] Dr A. Geikie on the History of Volcanic Action. 347 
and alternate with these eruptive masses, and are usually more or less 
altered towards the contact. These gabbros and other basic eruptive 
rocks are thus younger than the surrounding basalts of the plateaux. 
They probably availed themselves of older vents of the plateaux, 
rising more readily in these as points of weakness in the terrestried 
crust, and raising up the overlying bedded basalts in dome-shaped 
elevations. Whether or not any of these domes were disrupted at 
the summit, so as to allow of an outflow of basic rock at the surface, 
cannot be affirmed ; if any such outflow took place, it has been 
entirely removed by denudation. 
IV. In the fourth part a detailed account is given of the next 
great episode in the Tertiary volcanic history— the extravasation of 
a series of thoroughly acid rocks. The petrographical characters of 
these rocks show them to be divisible into two groups, well marked 
off from each other in mineral characters and in age. On the one 
hand, are compounds of turbid orthoclase and quartz, ranging from 
a flinty felsitic texture into crystalline granophyres, and even true 
granites ; these are the older rocks. On the other hand, come 
quartz-trachytes (with clear sanidine) and pitchstones. The modes 
of occurrence of the acid rocks are then narrated, with the local 
details of each district in which they occur. The great granophyre 
bosses of Skye, Mull, and Kum are shown to have broken through 
the basalt plateaux and the gabbro bosses, sending veins into these 
rocks and producing contact metamorphism in them. They have 
likewise taken advantage of older vents, segments of which can still 
be detected around them. Besides the bosses, the granophyres 
occur as massive intrusive sheets or sills, running for miles in the 
Jurassic strata or between these and the base of the basalt plateaux, 
mostly now removed. They are also found in the form of veins and 
dykes, which occur in prodigious numbers in some portions of the 
basic rocks. The granophyres, felsites, and granites are traversed 
by a younger series of basic dykes, and also by pitchstone veins, 
which, not being cut by these dykes, may be the youngest protru- 
sions of all. In Antrim bosses of trachyte and pitchstone rise 
through the plateau-basalts. Connected with the emission of vitreous 
sanidine lava is the rock of the Scuir of Eigg — a true superficial out- 
flow, and the only one of the acid series which now remains. As 
this rock has been poured into a river-bed eroded out of the surface 
