fi 
Prof. Grenville Cole—The Rocks of Rhobell-Fawr. 341 
another and more felspathic mass occurs, possibly a lava contem- 
poraneous with the ashes round it. Its specific gravity is 2°75. 
The great area for these puzzling rocks is, however, the high 
shoulder traversed by two romantic little passes, the Bwlch Gori- 
wared and the Bwlch Gwyn. ‘The latter spot, especially at sunset, 
is one of the most charming view-points in North Wales. Where 
the road from the base of Moel Hafod-Owen begins to rise and go 
over to Llanfachreth, the trachytic andesites come in intrusively 
among the slates. One can trace them along Foel Cae-poeth, and 
pick them up again at the Bwlch itself in a tor-like exposure on 
Moel-y-Llan. <A great amount of calcite has arisen in them, and 
ferromagnesian minerals are hardly traceable. The specific gravity 
of the rock near Foel Cae-poeth is as low as 2°70. 
On Moel-y-Llan the rock contains abundant small crystals of 
pyrite, which are now often represented by skeletal remains. De- 
composition-products, probably the stable oxide limonite, separated 
out along planes that, from their angles of intersection, seem to be 
those of the octahedron ; consequently we have now the black casts, 
as it were, of these cracks alone remaining, with a mere amorphous 
infilling between their intersections to represent the solid crystal. 
‘These dark skeletal meshes are certainly formed of planes, not of 
intersecting rods; and hence the above explanation seems the true 
one. 
Between the two passes, on the hummocky ridge, the flinty 
andesite weathers white like the Arenig eurites, and is in every way 
calculated to deceive the observer. On the high boss just west of 
the Bwlch Goriwared, the greener and softer character reveals itself 
when the rock is broken, and it has a specific gravity of 2-76. 
The pseudomorphs in this place point to hornblende as an original 
porphyritic constituent, which would mark off this example from all 
the other trachytic andesites of Rhobell. Skeletons of pyrite occur, 
recognisable only when compared with those of Moel-y-Llan, since 
the interstices are in this case coloured a pale chlorite-green. 
Then, down under the Coed Cwm-yr-Hglwys, close to Llan- 
fachreth, there is a very fine-grained pyritous rock, with a specific 
gravity of 2:84, probably belonging to this series; but it is so soft 
and fissile that it has been collected only to be laid aside again. 
Enough unsatisfactory material has already been described. 
LV. Tue Turrs anp Asnes. 
The great remaining masses of the Rhobell Fawr volcano are 
tuffs and ashes, the former containing in places blocks some 20cm. 
across; but the general character, despite the description in the 
Survey Memoir, is fine-grained in comparison with those of Snowdon 
or Cader Idris. 
On the south the ashes are in general less coarse, and take up the 
cleavage of the district, as may be noticed in Garth Fach. Near 
the summit of Moel Cors-y-garnedd they have been laid out in 
water, and contain sand-grains and argillaceous bands, passing in 
places into the grit that caps the hill. Similarly the ashes are gritty 
