nwy; this rises in Llyn Conwy, in the south-east, divides Carnarvon
from Denbigh (running nearly due north) for some 30 m., and falls into
the sea at Deganwy. The Seint (wrongly spelled Seiont) is a small stream
rising in Snowdon and falling into the sea at Carnarvon, to which it
gave its old name Segontium (Kaer Seint yn Arvon in the _Mabinogion_).
The Swallow Falls are near Nant Ffrancon (the stream of the Beaver or
Afanc, a mythological animal). Nant Ffrancon leads north-west from near
Capel Curig and Bettws y coed and past Bethesda, reaching the sea in
Beaumaris Bay. The lakes, numerous and occasionally large, include: Llyn
Peris and Llyn Padarn at Llanberis, north of Snowdon; Llyn Ogwen, north
of Glydyr Fawr; Llyn Cowlyd and Llyn Eigiau, both north of Capel Curig;
Llyn Llydaw, on Snowdon; Llyn Cwellyn, west of Snowdon; Llyn Gwynnant,
east of Snowdon; Llyniau (Nant y llef or) Nantlle, near Llanllyfni; Llyn
Conway.

The greater part of the county, including the mountainous Snowdon
district and nearly all the eastern portion of the promontory of
Lleyn, is occupied by rocks of Ordovician age, the Arenig, Bala and
Llandeilo series. These are dark slates and thin-bedded grits with
enormous masses of interbedded igneous rocks, lavas and ashes, the
product of contemporaneous volcanoes. At the base of Snowdon are Bala
grits and slates, above them lie three beds of felspathic porphyry,
which are in turn succeeded by a great mass of calcareous and sandy
volcanic ashes, while upon the summit are the remnants of a lava
sheet. The whole mountain is part of a syncline, the beds dipping into
it from the north-west and south-east.

Next to the Ordovician, the Cambrian rocks are the most important;
they are found in three separate areas; the largest is in the
north-west, and extends from Bangor to Bethesda, through Llyn Cwellyn
and Llanwada to the coast near Clynnogfawr. The second area lies west
of Tremadoc, which has given its name to the upper division of the
Cambrian system. The third forms the promontory south of Llanenga.
Cambrian slates are extensively quarried at Penrhyn, Llanberis and
Dinorwic. Pre-Cambrian schists and igneous rocks occupy a strip, from
2 to 3 m. wide, along the coast from Neirn to Bardsey Island. A very
small area of the Denbighshire Silurian enters this county near Conway
near the eastern border; it comprises Tarannon shale and Wenlock beds
with graptolites.