342 Prof. Grenville Cole—The Rocks of Rhobell-Fawr. 
under the corresponding bed on Rhobell Fawr,! while the grit 
(‘Garth Grit”) itself contains pebbles of lava washed down into it. 
Coarser tuffs, with unmistakable scoriz#, may be seen from the 
tumbled moorland below Cerniau, and across the Pabi, away to the 
crest of Rhobell Fawr. A section cut from one of these scoriz 
shows the felspar replaced entirely by epidote and chlorite, and the 
ferromagnesian constituent represented by mere black skeletons. 
Its specific gravity is 2°88. In general the tuff has a purple tint, 
and the scoriz weather away more easily than the tough interstitial 
ash, which is quite contrary to one’s experience on more recent cones. 
Here the whole mass has been so compressed, consolidated, and 
mineralized, that the looseness of the ash or the massiveness of 
the ejected blocks no longer exerts an influence, and the amount 
of loss by denudation depends mainly on compactness. The vesicular 
blocks of lava are eaten away, and the surrounding volcanic dust 
stands out as a meshwork of fine ribs. 
The highest rock on Moel Cors-y-garnedd, even above the grit, is 
a grey highly felspathic ash, with a specific gravity of 2°70. The 
plagioclases are well preserved, and this bed points to the intrusion 
and eruption of the trachytic andesites during the last phase of 
activity on Rhobell Fawr. The slate ash at the base of the Arenig 
series east of 'T'y-newydd-y-mynydd is also felspathic, like many on 
Cader Idris; but there are fragments of glassy lavas in it unlike 
anything on Rhobell, and it probably marks the next period of 
activity from a vent further to the east. 
There is also ash on Rhobell Fawr above the ring-like exposure 
of the grit; below, especially to the north and south, we descend 
over magnificent developments of Hornblende-Tuff. 
This rock has at times been styled a “ Hornblende-Porphyry ” ; but 
the microscope exposes its typically fragmental character. In the 
wide moorland from which Rhobell Ganol, Moel Gron, and the bold 
peak of Rhobell-y-big rise, hornblende-tuffs and ashes are every- 
where conspicuous, until we find their most striking representatives 
in Rhobell-y-big itself. A little to the east, down on the nearest 
tributary of the Mawddach, these deposits may be seen in contact 
with fossiliferous Lingula Flags. On the whole, they form a basal 
feature of the voleano, and their coarseness at Rhobell-y-big suggests 
a local centre of eruption north of the main crater. Possibly the 
andesite with porphyritic zoned hornblendes, previously described 
from the N.W. spur of Rhobell, may have acquired its hornblende 
from these tuffs, which were traversed by it in its ascent. 
The scorize and lumps of lava in the hornblende-tuffs are merely 
basaltic and trachytic andesites with altered pyroxene, resembling 
those described above as massive rocks. An ejected block from 
Rhobell Ganol, containing much epidote, has a specific gravity 
of 2°96. Small irregular brown hornblendes occur in some of the 
fragments on the south of Rhobell Fawr; but I find in these frag- 
ments as a whole a repetition of the lavas of the area, and the 
1 See Cole and Holland, ‘‘Structure of Rhobell Fawr,’? Guot. Mac. 1890, 
pp. 448 and 452. 
