464 
THE TERTIARY VOLCANOES 
BOOK VIII 
these strata and the overlying volcanic series, or along any other plane of 
weakness in the terrestrial crust. In this way arose the multitudinous 
sills or intrusive sheets. 
When the great volcanic plateaux had been built up to a thickness of 
several thousand feet, another remarkable episode in the history occurred. 
At certain points large bodies of coarsely crystalline basic rocks were pushed 
into and through the plateaux -basalts, upraising them in dome-shaped 
elevations, and ultimately solidifying as dolerites, gabbros, troctolites, 
picrites, etc. There is reason to believe that the points of extravasa- 
tion of these materials were mainly determined by the positions of 
the larger or more closely clustered vents of the plateau -period, where 
points of weakness consequently existed in the terrestrial crust. Bising as 
huge bosses through such weak places, the gabbros and associated rocks 
raised up the overlying bedded basalts, and forced themselves between them, 
forming thus a fringe of finer-grained intrusive sills and veins around the 
central handed and amorphous masses of more coarsely crystalline material. 
Whether, in any of these vast domes of upheaval, the summit was disrupted, 
so as to allow the basic intrusion to flow out as lava at the surface, cannot 
now be told, owing to enormous subsequent denudation. 
The next chapter in the chronicle shows us that probably long after the 
eruption of the gabbros, when possibly all outward symptom of volcanic 
action had ceased, a renewed outbreak of subterranean activity gave rise to 
the protrusion of another and wholly different class of materials. This 
time the rocks were of a markedly acid type. They included varieties that 
range from obsidians, pitchstones, flinty felsites and rhyolites, through 
porphyries and granophyres, into compounds which cannot be classed under 
any other name than granite. These masses likewise availed themselves of 
older vents in the plateaux, and broke through them. They now form huge 
conical hills, which, in their outer aspect, and even to some extent in their 
inner structure, recall the tracliytic puys of Auvergne. But the granophyres 
not only ascended through the basalt-plateaux and the gabbro-bosses ; they 
sent into these rocks a network of veins, pushed their way in huge sheets 
or sills between the strata below, and actually incorporated a considerable 
proportion of the basic materials into their own substance. Around the 
bosses of gabbro and granophyre, the bedded basalts have undergone con- 
siderable contact-metamorphism. 
The gabbro and granophyre bosses of the Inner Hebrides demonstrate 
with singular force how unreliable petrographical characters are as a test’ 
of the relative age of rocks. Ho one, looking at hand-specimens of these 
rocks, or even studying them in the field, would at first suspect them to 
be of Tertiary date. They closely resemble rocks of similar kinds in 
Palaeozoic and even Arclueau formations. Yet, of their late appearance in 
geological time, there cannot be any possibility of doubt. 
After the uprise of the granophyre, and the injection of the network of 
felsitic veins, there came once more a period of terrestrial convulsion, like 
that of the earliest basic dykes, but of less intensity. Again, the crust of 
