10 A. R. C. SELWYN ON THE 
tions of the upper part of the metamorphic group, which I have elsewhere provisionally 
designated the volcanie group, it being in certain parts of its distribution very largely 
made up of rocks which, while clearly interbedded with the ordinary stratified deposits 
of the formation (clay, slates, quartzites and sandstones), have all the mineralogical charac- 
ter of igneous rocks, but in a highly decomposed and altered condition. 
Specimens of most of the varieties of these rocks are exhibited and it will be seen that, 
in their more than ordinarily decomposed and altered state, they essentially correspond with 
rocks which are almost universally held to be of igneous, eruptive or irruptive origin, 
while those of the bedded and fragmentary character seem very plainly to point to con- 
temporaneous volcanic action as the true cause of their formation, whether as agglomerates, 
ash beds, mud coulis, lava flows, or Intrusive dykes and masses. 
In support of this view I would submit that, on no other supposition of the origin 
of the rocks referred to, is it easy to understand their close association with wholly 
unaltered argillites ; stated as follows, page 600, Geology of Canada: “ A great proportion of 
the argillites have, however, undergone no apparent change, but are found in the imme- 
diate vicinity of the serpentines and steatites, earthy in texture, and with no appearance of 
alteration.” 
I would also submit that neither a schistose nor a bedded structure can be accepted as 
proof of anon-igneous or yolcanic origin, and that a once massive laya flow, whether augitic 
or felspathic, is as likely, through pressure and metamorphism, to assume a schistose struc- 
ture as are ordinary sedimentary strata. It is, lam aware, notin accordance with generally 
received ideas on the nature of ancient igneous rocks to suppose they can be schistose and 
stratified, especially so in America where volcanic agency, in the earlier geological periods, 
has been almost entirely ignored, and all those rocks which, by their microscopic characters 
and chemical composition, and by their geological associations and relations, point to volcanic 
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agency as the cause of their formation, have been said to be “ not igneous but metamorphic in 
origin,” a description which it seems to me is decidedly self-contradictory. 
In a recent paper read by Professor Bonney before the Geological Society of London 
“On the nodular felsites of the Cambro-Silurian Bala Group of North Wales,” he shews that 
a green schist is only an amygdaloidal felsite, in which after cleavage a secondary (micaceous) 
mineral has been deposited along the cleavage planes; and these cleaved felsites show dis- 
tinct flow structure, proving them to be ancient bedded lavas. On the same subject we find 
the following remarks in “ Volcanoes what they are and what they teach,” by Professor 
Judd, 1881 :— 
“We know that during the Pre-Cambrian periods volcanic outbursts took place, traces 
of which are found both in North and South Wales, in the Wrekin Claim in Shropshire, in 
Charwood Forest, and in parts of Scotland and Ireland.” 
In Cambro-Silurian times we have abundant proofs, both in North Wales and the Lake 
district, that volcanic action, on the very grandest scale, was taking place during the Arenig 
and the older portion of the Llandeilo periods, and again during the deposition of the Bala 
or Caradoc beds. The lavas, tuffs and volcanic agglomerates ejected during these two 
periods have built wp masses of rock many thousand of feet in thickness. Snowdon and 
Cader Idris, among the Welsh mountains, and some of the higher summits of the Lake 
districts have been carved by denudation from. the vast piles of volcanic materials ejected 
during these periods. 
