File:The Quarterly journal of the Geological Society of London (13205718965).jpg

1846.. SEDGWICK ON THE FOSSIL SLATES OF N.WALES, ETC. 137
discover a single fossil, not even so much as a fucoid; though I be-
lieve the rocks belong to a fossiliferons group. A similar group
of dark slates breaks out farther south, and is at length interrupted,
and broken up, by the great erupted porphyries of the Rivals. Asso-
ciated with these rocks we find some irregular beds or masses of
iron ore, sometimes highly magnetic, and with a remarkable piso-
litic structure. They are probably contemporaneous; and the beds
along which they run sometimes pass into the condition of a load-
stone, exhibiting a true polarity. I should not have noticed these
phasnomena had they not been exhibited in several other places
among beds which are, I believe, exactly on the same parallel. On
the east side of the great intrusive rib of porphyry, above mentioned,
and towards the base of the higher mountains of the Carnarvon
chain, are several other alternations of slates and contemporaneous
trappean rocks. They are, I believe, in symmetrical undulations,
which strike nearly with the beds ; but the country is low, and much
covered with the drift of the higher mountains, as well as with great
masses of a northern drift, which (as is well known) contains ma-
rine shells to the height of more than 1200 feet.
Again, to the east of this low country is another and larger rib of
porphyry, extending from the country near Llanllyfni to the hills
north of the foot of Llanberis lake (see the Map)*. In position and
structure it very nearly resembles the more western rib of syenitic
porphyry. On the other hand, the beds of its upper surface are of
such remarkable structure, and offer such apparent passages into the
overlying slates, that I cannot, in all the quarries, separate one for-
mation from the other ; I am compelled therefore to consider the
date of this enormous rib of porphyry as doubtful. But, at any rate,
it leaves undisturbed the general symmetry of the great undulating
east and west section through the Carnarvon chain : for a section
from the Menai near Bangor through the Penrhyn quarries, and
thence to Glyder Fawr, presents the same undulations, and the
same succession of stratified masses, with another section from Car-
narvon across the two ribs of porphyry to the top of Snowdon ; or,
still farther south, to the top of Moel Hebog.
Section I.
Menai Straits to Glyder Faivr on the Carnarvon Chain.
Horizontal line 9 miles. E.S.E.
Glyder Fa^vr.
W.S.W.
Menai Straits,
a. Black slates. d. Penrhyn slates.
5. Porphyry with traces of bedding. e. Coarse greyvvacke.
c. Porphyry and undulating slates". /. Slate and porphyry.
To the east of the second great rib of porphyry commences the
great zone of the Carnarvon slates, ending with the quarries of Pen-
rhyn, Llanberis, Llanllyfni, &c. They alternate with many bands

• The Map referred to is that which accompanies Professor Sedgwick's former