338 Prof. Grenville Cole—The Rocks of Rhobell-Fawr. 
mountain, setting aside the hornblende crystals that occur so freely 
in the tuffs. Augite occurs, as we shall see, as the prevalent ferro- 
magnesian constituent of the massive rocks and also of the scoriz. 
The augite-aphanites, or dolerites, do not contain olivine, and are 
the holocrystalline representatives of fairly basic andesites. One of 
the freshest examples (a third of a mile north-east of the summit of 
Rhobell Fawr, and across the wall) has a beautiful ophitic structure. 
The plagioclases, by the test of symmetrical extinctions, prove 
to be oligoclase, with here and there an albite, and are excellently 
preserved ; but titanic iron ore is freely scattered, and the specific 
gravity of the mass is 2°89. In the steeply descending enclosed 
area half a mile south of the summit there is a band of augite- 
aphanite intersected by abundant veins of compacter material. This 
mass is more basic, with labradorite and titanic iron ore, and a 
specific gravity of 2:97. The ophitic structure, so common also 
in the Cader Idris ‘“‘ greenstones,” is again conspicuous; but the later 
stage of consolidation has been hurried, and the ophitic areas thus 
lie scattered in a fine-grained groundmass. In fact, it is an un- 
usually striking example of distinct ophitic crystals of augite, which, 
moreover, are often simply twinned. The included felspars run in 
from the surrounding groundmass, and hence the ophitic areas were 
formed in situ, and not floated up from a shattered and pre-existing 
mass. 
A third specimen, on the fringe of the area, south-east of Allt- 
lwyd farm on the upper Mawddach, is continuously ophitic. Pyrite 
is developed in it, a characteristic mineral of the whole district. 
The intrusive mass to the south of the summit of Moel Cors-y- 
garnedd is a handsome augite-aphanite with scattered porphyritic 
pyroxene; its specific gravity is 2°98. It might prove to be another 
outcrop of the mass that appears in the enclosed area south of 
Rhobell Fawr. 
There are some bosses of compact aphanite, coloured yellow- 
green by epidote, on the south of the volcano, between the ashes of 
Bryn Bris and Garth Fach; there is a small exposure below the 
footpath east of the summit of Moel Cors-y-garnedd; and in the 
valley of the Geirw, between Rhobell Ganol and the north-west 
spur of Rhobell Fawr, there are three or four places where aphanites 
come up through the hornblende-ashes or the Lingula Flags. One 
of these rocks, almost as fine-grained as an andesite, has a specific 
gravity of 2°98, which is accounted for by the copious development 
of yellow epidote, one of the commonest minerals in basic igneous 
rocks throughout N. Wales. These are all the occurrences of un- 
doubtedly holocrystalline rocks with which I am acquainted in 
this area. 
IJ. Tue Basautic ANDESITES. 
There are, however, a number of andesites, some of which are 
intrusive. They are all somewhat uninteresting to the eye; but they 
divide themselves on closer examination into augite-andesites of a 
basaltic character, doubtless once richly dark in colour like those of 
the Nahe or of Selmeczbanya; and a series of highly felspathic 
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