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1846.. SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 139
appear here and there in the beds of this trough, first breaking out
on the higher part of the brow ascending from the Penrhyn quarries
to Llyn Ogvven. Along this line we have a Ibssiliferous zone about
half a mile wide, and it ranges through the higher crests of Snowdon
to Moel Hebog, as stated in former papers*. In many extensive
tracts it is true that fossils disappear, especially where contempora-
neous porphyries and trappean shales are most abundant ; but the
whole series of undulations, continued from the high crests here no-
ticed, over the Berwyns, and to the edge of Shropshire, belong to
one great physical group, the whole of which (where the conditions
are favourable to the development of animal life) is fossiliferous.
Such are the sections from the Menai to the crests of the Car-
narvon chain ; and I cannot estimate the thickness of the beds, be-
fore we reach the fossiliferous trough above noticed, at less than six
or eight thousand feet.
But there is one great imperfection in these sections — they have
no well-defined base ; and we have no evidence on which to estimate
the thickness of deposits which may have been interpolated between
the hypozoic group and the dark, earthy slates of Carnarvon and
Bangor. Moreover, it is almost impossible to form any correct esti-
mate of the thickness of the masses occupying the low country be-
tween the Menai and the western flank of the Carnarvon chain ; we
must therefore seek for better CN^idence in other sections
Section III.
Near Tremadoc.
Horizontal distance 8 mUes. . ■"' "
S. by W.
a. Lingulabeds.
b. Black slates with Fucoids and Trilobites.
c. Trilobites, &c.
I have stated in my former papers that a great dislocated group of
slates and porphyries (not to be mineralogically distinguished from
the general mass of the Carnarvon chain) occupied the promontory
south of Tremadoc, and was continued northward till it abutted
against the south flanks of Moel Hebog and Moel Ddu, a few miles
south of Beddgelert ; but I had not seen this group since 1831. Its
beds first dip nearly north, and then bending round toward the
western side of the great estuary, called Traeth Mawr, the beds dip
about N.N.E. Before rising to the flanks of Moel Hebog and Moel
Ddu, they reach a great elevation, and become in some places almost

• See Proceedings of the Geological Society, vol. iii. p. 548.