i 
Prof. Grenville Cole—The Rocks of Rhobell-Fawr. 348 
conspicuous crystals of hornblende lie in the ground between them, 
and are derived from some deep-seated source. 
The conditions that give rise to amphibole being hydrothermal, 
rather than those that prevail in the ordinary cooling of a lava, it is 
not necessary to suppose that in this instance an amphibolite has 
been shattered by the explosions and its remains mingled with 
the normal lavas; we may conceive that the brown hornblende 
developed porphyritically as a deep-seated constituent from the local 
magma, but that augite took its place when the same magma 
consolidated nearer the surface. In that case, considering the 
enormous abundance of the crystals, it is remarkable that we so 
seldom find them included by flotation in the lavas from which they 
are supposed to have developed. Iam inclined to think that a rock 
rich in amphibole did actually exist beneath this area, perhaps a 
true amphibolite, and that it was traversed and broken up by the 
eruptions of basaltic andesite. These eruptions became less uniform 
in character towards the close of activity, giving us, particularly 
on the west, the grey trachytic andesites, and in places the hand- 
some ophitic aphanites, which are often of a more basic character. 
The amphibole of the tuffs is accompanied, however, by pyroxene, 
which is often quite as abundant. The two minerals are found 
grouped together, and evidently once formed part of the same rock. 
SECTION oF AsH FROM SuMMIT or RuoBELL-y-BIG, MERIONETHSHIRE, X 5. 
a, Augite, pale yellowish, with cracks and ovoid areas filled with green decomposition- 
products. A large cross-section is conspicuous above. 
h, Hornblende, rich brown. The large lower one includes small granular pyroxene, 
but also has several almost colourless areas, across which the cleavage runs, and 
which may result from the substitution of secondary amphibole for some included 
mineral. 
c, Hollows filled with chlorite, the right-hand one containing small secondary 
amphibole, and the left-hand one a border of yellow epidote. 
