CHAP, xiii 
BALA ERUPTIONS OF CAERNARVONSHIRE 
2 
and enclosed one within the other (Fig. 55). The general push indicated 
by them points to a inoveineut from the westward. Turning round from 
the crags, and looking towards the west, we see before us on the other side 
of the deep vale of Llyn Cwellyn, at a distance of little more than three 
miles, the great dome-shaped Mynydd-mawr, which, there is every reason to 
believe, marks one of the orifices of eruption. It might in this way he 
pi'aeticable to obtain information regarding even some of the vents that still 
lie deeply buried under volcanic or sedimentary rocks. 
That these felsites were poured forth in a glassy condition may be 
inferred from the occurrence of the minute pcrlitic and spherulitic forms so 
characteristic of the devitrihcation of once vitreous I’ocks. Mr. Eutley was 
the first who called attention to this interesting proof of the close resemblance 
between Palfeozoic felsites and modern obsidians, and other observers have 
since confirmed and extended his observations.^ 
i IG. 55. — Flow-structure in tlie lowest fetsite on tlie track from Llanbcri.s to tlie top of Snowdon. 
Length about 4 feet, height 2^ feet. 
Another remarkable aspect of the felsites is that nodular structure so 
often to be seen among them, and regarding the origin of which so much 
has already been written. I agree with Professor Cole and Mr. Harker in 
looking upon the “ nodules ” as derived from original splierulites by a process 
of alteration, of which almost every successive stage may be traced until the 
original substance of the rock has been coir verted into a flinty or agate-like 
material. If this be the true explanation of the structure, some of the 
original lavas must have exhibited jierlitic and spherulitic forms on a 
gigantic scale. There can, I think, be little doubt that this peculiar structure 
was very generally misunderstood by the earlier observers, who naturally 
looked upon it as of clastic origin, and who therefore believed that large 
beds of rock consisted of volcanic conglomerate, which we should now map 
as nodfilar felsite (pyromeride).'^ 
^ Quart Journ. Geol. i^oc. vol. xxxv. (1879), p. 508. 
Another source of error may probably bo traced in the occasional brecciated structure of the 
felsites, which has been mistaken for true volcanic breccia, but which can be traced disappearing 
into the solid rock. Sometimes this structure has resulted from the breaking up of the lenticles 
of flow, sometimes from later crushing. 
