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Prof. Grenville Cole—The Rocks of Rhobell-Faur. 339 
rocks, on the other hand, which I shall class here as Trachytic 
Andesites. Probably alteration has deepened the original grey tint 
of many of the latter, and lightened the blackness of many of the 
former series, intercalating specks of epodite and chlorite, until the 
whole group of andesites has become monotonously alike. 
Some, however, of the more basic series are heavy and compact, 
and yellow-green through abundant epidote. One of these, on the 
“summit of the picturesque bluff of Graig Fach, contains soft dark 
grey pseudomorphs, which seem under the microscope to be after 
pyroxene. This suggestion is borne out by several blocks gathered 
from the tuffs, and by the grey-green andesite of the cliff west- 
north-west of Ty-mynydd-y-newydd, a rock that looks merely like 
a baked ash in the field. Under the microscope it shows porphyritic 
augites here and there; but the majority have been reduced to 
patches of epidote and chlorite. This rock, one of the lowest on 
the east flank of the volcano, has a specific gravity of 2-95. 
Then, among the hornblende tuffs of Rhobell-y-big, a mass of 
porphyritic augite-andesite occurs, successfully imitating the com- 
pacter tuffs to the naked eye. The augite is here in excellent 
preservation, and is simply twinned; the felspathic constituent, 
however, is nearly lost. In the groundmass a number of hexagonal 
pseudomorphs furnish one of those provoking suggestions of nephe- 
line which seem destined to remain nothing better than suggestions. 
The examination of early Paleeozoic lavas is sometimes more irritating 
than conclusive. 
Hard by, south of the fine crest of Rhobell-y-big, we have a 
grey-green and almost flinty lava with porphyritic augites, many of 
them entirely replaced by granular epidote. Those that are preserved 
show a very marked zonal structure, and the invasion of epidote has 
attacked them in the outermost zones. This rock has a specific 
gravity of only 2°83. 
A scoriaceous lava, one of very few, on the descent to the road 
south of the summit of Rhobell Fawr, with its cavities full of chlorite, 
zeolites, calcite, and chalcedony, is basic enough to be referred to 
this division of the andesites. 
Coming farther south, there is an andesite, rich in epidote, at the 
north-east end of Bryn Bras; the porphyritic crystals, soft and dark 
grey, are mere green pseudomorphs under the microscope, with little 
epidote granules in a chlorite ground ; but their cross-sections in the 
specimen show them to have once been pyroxene. 
So far, hornblende does not seem to be a constituent of the basaltic 
andesites of Rhobell, though occurring freely in the tuffs. But high 
up on the north-west spur of the mountain, east of Ty-canol, and 
west of the hollow of the Geirw, there is a broad exposure of 
scoriaceous lava which contains abundant hornblendes. In the 
field Mr. Marshall and myself referred it to the hornblende ashes, 
but fragments of lava were not apparent in the groundmass. The 
zoning of the hornblendes was the most pronounced feature; and 
this, under the microscope, is seen to be connected with their 
alteration. The lava is an amygdaloidal augite-andesite, the por- 
