CAMBRIAN SERIES IN N.W. CAERNARVONSHIRE. 319 
Conclusions. 
We see, then, that the rock in the quartz-felsite massif exhibits 
every characteristic of an igneous origin; we see also that fragments 
closely corresponding with it occur again and again in the overlying 
rocks, and that theserocks, as a rule, arecomparatively little changed— 
the evidence of alteration being illusory, and quite ordinary slates or 
grits in some cases interposing between the quartz-felsite and the 
supposed highly metamorphosed rocks. We must then refuse to these 
Caernarvonshire “ porphyries” an origin different from that of other 
igneous rocks of similar composition, and cease to quote them as-ex- 
amples of what extreme metamorphism can effect. 
I may repeat again that, allowing for slight mineral changes 
brought about by the agencies to which all rocks have been exposed 
in the long lapse of ages (such as devitrification, the formation of 
viridite, &c.), there is no difference of any importance, so far as I can 
see, between these quartz-felsites and comparatively modern rhyo- 
lites; and if I could prove that a base still remained undevitrified, I 
would give them the latter name. ‘That they were rhyolites in pre- 
Cambrian times I have no doubt. 
Further examination will probably discover more agglomerates, 
and perhaps further subdivide the lava-flows. which certainly seem at 
present of exceptional thickness and extent*. Whether volcanic 
action continued during the time when the conglomerates of Tair- 
ffynnon &c. were formed is uncertain. The lapilli in them are in 
some cases not waterworn ; but then they may have been derived from 
the destruction of cones of scoria, which, no doubt, rose among and 
even upon the neighbouring lava-flows. Their lightness might cause 
them to float for a while, and so escape the rounding which has 
befallen some of the other materials. The uppermost beds at 
Bangor (and to these, perhaps, we may add the Minffordd rock) 
certainly seem to indicate volcanic action} ; but the ejecta are very 
different, and the outbursts were probably very local. 
The evidence which has been brought forward warrants, I think, 
our proposing the following general ascending order for the group 
near Bangor between the quartz-felsite and the Cambrian conglome- 
rate :—(1) felsitic grit and fine conglomerate; (2) green slates{; 
(3) conglomerates with interbanded green grits passing into slaty 
grits—Tair-ffynnon to Perfyddgoed; (4) another group of gritty 
and conglomeratic beds ending with the conglomerate of Cae Seri; 
(5) Mynffordd rock and upper green breccias, grits, and slates. 
Doubtless there are gaps in this grouping, and changes may hereafter 
* The larger massif is about thirteen miles long and two at greatest breadth. 
The smaller on the map appears not quite two thirds the above dimensions, but, 
as will be seen hereafter, is really much less. Examples of very extensive lava- 
flows, so far as I remember, are much more common in basic than in acid 
rocks. 
+ Quart. Journ. Geol. Soc. vol. xxxiv. p. 146. 
t It is almost impossible to give precise names to these bedded rocks. One 
finds almost every variety from fine grit to fine, but imperfectly cleaved, slates. 
Our petrological nomenclature always seems to me a little too restricted in re- 
gard to this group of rocks. 
