344 Prof. Grenville Oole—The Rocks of Rhobell- Fawr. 
Sometimes brown hornblende has developed paramorphically and 
sporadically within a large pale pyroxene; but much of the horn- 
blende looks original and distinct. These crystals remind one so 
strongly of the Peridotites of Schriesheim or Loch Seye that one 
expects to find included olivine ; but only a few green ovoid pseudo- 
morphs in one specimen give direct countenance to this idea. The 
crystals are much torn and shattered; the cracks of the pyroxene 
have been filled up by chlorite, or by a very finely divided granular 
material; the hornblende has, however, been restored by a pale 
amphibole in optical continuity.!_ The hornblende crystals have also 
in places been extended by secondary amphibole, in flame-like pro- 
trusions, into the surrounding ash, and a number of actinolitic 
needles run through the altered eroundmass. 
In some of the hornblende ¢rystals there are suspicious ovoid areas 
of a very pale green tint, across which the mineral is optically con- 
tinuous. ‘These may represent .places from which olivine has been 
entirely removed, the gap being steadily filled in by the familiar 
pale secondary amphibole. A comparison of this structure with 
that of the olivine-bearing rocks of Menheniot, Schriesheim, ete., 
makes the suggestion seem at any rate not extravagant. 
V. Tuer Grits. 
The detailed examination of the grit-bands proves their aqueous 
origin as clearly as is done by the larger pebbles in the field. It is 
rather difficult to know where all the quartz-grains came from— 
certainly not from any disintegration of the volcanic rocks of Rhobell. 
A few pebbles of compressed quartzite seem to point to the Harlech 
Grits as a source of origin, or even to the pre-Cambrian rocks from 
which those masses were derived.?. The pebbles of lava are probably 
local, resembling the trachytic andesite series; and the abundant 
isolated plagioclases are additional evidence that the more felspathic 
eruptions occurred at the close of the volcanic action. 
The most interesting constituents of the grit-beds are the ovoid 
black and grey bodies that show a concentric structure on weathering. 
Mr. Holland and myself? referred this structure to contraction; but 
microscopic sections show that it is in reality fundamental. ‘These 
bodies, have, in fact, a rippled concentric mode of growth, and their 
layers appear transparent brown, colourless, or faintly green. Some 
of them include angular grains of quartz-sand, which show that they 
have arisen somehow in a sedimentary rock. On boiling the dark 
mass in hydrochloric acid, the greater part dissolves with effer- 
vescence, colouring the acid intensely yellow. A fine residue of 
quartz-sand, mica, and minute argillaceous particles, alone remains. 
These curious bodies may be referred, then, to the beds of pisolitic 
iron ore which were forming on the spot at the opening of the 
Arenig epoch; and they are, in fact, isolated concretions, often of 
large size, rolled out from those deposits. The best examples of them 
1 @f. Marshall Hall, Gzon. Mac. 1889, p. 480. 
* Of. Jennmgs and Williams, ‘‘On Manod and the Moelwyns,’’ Quart. Journ. 
Geol. Soc. vol. xlvii. p. 378. 8 Loc. cit. p. 449. 
