CHAP. XII 
A RENIG ER UP TIONS—MERIONE THSHIRE 
183 
the “ lava-streams of Llaiideilo age,” in Merionethshire, are named “ telstone,” 
“felspar -porphyry,” “ felstone- porphyry,” “felspathic- porphyry,” and “cal- 
careous amygdaloid.” 
The most interesting feature which my own slight personal acquaintance 
with the region has brought before me is the clear evidence of a succession 
from comparatively basic lavas in the lower part of the group to much more 
acid masses in the higher part. In the Survey map numerous sheets of 
intrusive “ greenstone ” are shown traversing tlie Lingula Flags, Tremadoc 
slates, and lower part of the volcanic group along the northern slopes of 
Cader Idris. The true intrusive nature of much of this material is clearly 
established by transgressive lines of junction and by contact-metamorphism, 
as well as by the distinctive crystalline texture of the rocks themselves. 
But the surveyors were evidently puzzled by some parts of the ground. 
Sir Andrew Ilamsay speaks of “ the great mass of problematical vesicular 
and sometimes calcareous rock which is in places almost ashy -looking.” 
After several oscillations of opinion, lie seems to have come finally to the 
conclusion that this vesicular material, which occurs also in the upper part 
of the mountain, passes into, and cannot be separated from, the undoubted 
intrusive “ greenstones.” ^ 
The true solution of the difficulty will be found, I believe, in the 
recognition of a group of scoriaceous lavas among these greenstones. The 
presence of a cellular structure might not be sufficient to demonstrate that 
the rocks in which it appears are true lava-beds, for such a structure is far 
from unknown both among dykes and sills. But in the present case there 
is other corroborative testimony that some of these Cader Idris araygdaloids 
were really poured out at the surface. Below Llyn-y-Gadr — the dark tarn 
at the foot of the vast wall of Cader Idris — the beds of coarse voleanic 
conglomerate (h in Fig. 48), to which I have already alluded, are largely 
composed of blocks of the vesicular “ greenstones ” on which they lie. 
These “ greenstones.” moreover, have many of the most striking characteristics 
of true lavas (a in Fig. 48). They are extraordinarily cellular ; their upper 
surfaces sometimes present a mass of bomb-like slags with How-structure, 
and the vesicles are not infrequently arranged in rows and bands along 
the dip-planes. 
A microscopic examination of two slides cut from these rocks shows them 
to be of a trachytic or andesitic type, with porphyritic crystals ot a kaolinized 
felspar embedded in a microlitic groundmass. The rocks are much impreg- 
nated with caleite, which fills their vesicles and ramifies through their mass. 
A few miles to the east some remarkable felsitic rocks take the place 
of these vesicular lavas immediately below the pisolitic iron ore. I have 
not determined satisfactorily their relations to the surrounding rocks, and 
in particular am uncertain whether they are interbedded lavas or intrusive 
sheets. Ur. F. H. Hatch found that their microscopic characters show a close 
resemblance to the soda-felsites described by him from the Bala series of 
the south-east of Ireland. 
1 Mcni, (Jcol. Sure. vol. iii. Slid edit. p. 36 ; see also ]ip. 31, 32. 
