LOWER PALAOZOIC ROCKS OF ANGLESEY. 581 
on the mainland are the limestones, quartzites, hornstones, felspathic 
shales, and flinty slates, which are so prominent in the Anglesey 
Pebidian ? Where, on the other hand, shall we find in Anglesey any 
thing corresponding to the cleaved slates of Llanberis? To say that 
they are present, but metamorphosed into something else, is a pure 
assumption. Besides, we are justified in demanding proof that 
argillaceous slates can be altered into felspathic shale, quartzite, and 
limestone. 
2. While the Pre-Paleozoic rocks are quite unlike known Cam- 
brian strata, they closely resemble formations which in other parts 
of Britain are known to underlie Lower Cambrian beds. Several 
years ago I called attention * to the affinities between the slaty 
series and the Pebidian strata of St. Davids, which certainly underlie 
Harlech rocks, as the most recent critics admit +. The resemblance 
to certain Salopian types is also well marked. 
3. Those who hold that the schists of Anglesey are of Cambrian 
age can hardly, I think, have considered the difficulties which their 
hypothesis involves. These schists were in their present state of 
metamorphism before the deposition of the Ordovician rocks. We 
are therefore called upon to believe ¢ that, during the Upper Cam- 
brian period, the Lower Cambrian rocks sank thousands of feet, 
were covered in by a great accumulation of strata, were meta- 
morphosed into gneiss and schist, were raised above the sea-level, 
and were not only stripped of the Upper Cambrian masses, but 
were themselves deeply eroded by denuding agencies. 
4, According to Sir A. C. Ramsay § there must have existed near 
the Caernarvonshire area, during Lower Cambrian times, “an older 
rocky land,” “hilly or even mountainous,” which supplied to the 
Cambrian deposits granitoid materials ||, ‘purple and black slates, 
quartz-rock, felspathic traps, quartz porphyries and Jaspers.” But 
rock-masses still visible in Anglesey and on the adjoining mainland 
could have furnished all these varieties, except the black slates. 
Which, then, is the more probable supposition—that the altered 
rocks of Anglesey are the eroded fragments of a Pre-Cambrian region, 
or that Sir Andrew Ramsay’s Pre-Cambrian masses have been entirely 
swept away, and a portion of the Lower Cambrian series has been 
subsequently metamorphosed into a wonderfully close mimicry of 
the vanished land? 
But, whatever estimate we may form of the evidence here adduced 
for the Archzean age of the less-altered Pre-Ordovician rocks, 1¢ will 
be difficult to resist the proof which the Tywyn sections furnish with 
respect to the gneissic series. The green slaty rock, which on the 
old hypothesis is Lower Cambrian, contains numerous rounded frag- 
ments of the granitoid rock which abounds in Anglesey. Such 
evidence needs no further comment. 
* See Quart. Journ. Geol. Soc. vol. xxxvii. p. 211. 
t Dr. A. Geikie, zb7d. vol. xxxix. p. 286; Prof. J. F. Blake, 2bzd. vol. xl. p. 294. 
t This argument, of course, proceeds upon the popular hypothesis that 
regional metamorphism takes place only at great depths. 
§ Geol. N. Wales, 2nd ed. p. 196. || Lbid. p. 19. 
