~S 
Geological Society of London. 287 
present paper dealt with the nature of the eruptions that took place 
in this area and the characters of their products at successive strati- 
graphical horizons. The best. exposures occur, as is well known, 
upon the northern slopes. 
The lowest evidence of contemporaneous volcanic activity is to be 
found at the Penrhyn-gwyn slate-quarry, where a somewhat coarse 
bed of tuff, with slate-fragments and abundant felspar-crystals, 
occurs above an andesitic sheet. Similar slate-tuffs are repeated up 
to the base of the great cliff of Cader Idris, with intervening layers 
of normal clayey sediment. On the whole, the tuffs and ashes 
become more highly silicated as the upper levels are reached, and 
they terminate on the southern slopes in beds with fragments of 
perlitic and devitrified obsidian, such as are found under Craig-y 
Lilam. On Mynydd-y-Gader the intrusive dolerites have altered the 
ashes into hornstones; in places, moreover, they have become 
jointed into distinct columns. Fragments of andesitic glass as well 
as trachyte are recorded. 
The “pisolitic iron-ore” of the Arenig beds appears to have 
resulted from the metamorphism of an oolitic limestone, as in the 
case of the Cleveland ore described by Mr. Sorby, and that of 
Northampton described by Prof. Judd. The grains still give 
evidence under crossed Nicols of their having been built up of 
successively deposited concentric layers. The calcite so freely 
developed in the hollows of the underlying rocks may have been 
largely derived, during metamorphic action, from the destruction of 
similar thin limestone seams. No true lava-flows occur among 
these tuffs and sediments, a fact that implies comparative remoteness 
from the volcanic centre; and the important masses of intrusive 
matter represented upon the maps are themselves largely composed 
of the products of explosive action. ‘The numerous sheets of ophitic 
dolerite, aphanite, and altered andesite, that lie, seemingly inter- 
bedded, on the northern slopes, were probably intruded when the 
associated rocks were already weighed down by much superincum- 
bent sediment. A common character of these basic sheets is the 
development of small colourless crystals of epidote. 
The most striking mass upon the mountain is the main “ felstone ” 
(eurite) of the wall, which proves to be minutely “ granophyric,” 
and of very uniform grain throughout. An analysis by Mr. T. H. 
Holland shows 75 per cent. of silica. This vast intrusive sheet is 
regarded as perhaps of no later date than the Llandeilo lavas of 
Craig-y-Llam, and as a forerunner of the volcanic conditions that 
prevailed in Bala times throughout North Wales. 
The stratigraphical horizons, as shown on published sections, 
would throw a great part of the tuffs and ashes described into the 
Tremadoc beds, or even lower, in contradiction to the generally 
accepted statement that volcanic activity began in Arenig times. 
While this point can only be settled by detailed mapping on the 
basis of the new six-inch survey, the authors incline to the belief 
that the eruptions in this area broke out in the Cambrian rather 
than the Ordovician period. 
