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Very little is at present known of the Ordovician history of north-
eastern British Columbia. A boreal sea of this time is recorded by lime-
stones on Halfway River, west of Mount Wright (See Chapter III), and it
doubtless spread over a wide area on the site ot the Rocky Mountains and
Foothills and perhaps even over a part of the Plains. Other Ordovician
seas may have inundated this part of the continent, but their recognition
awaits further exploration.

The Silurian history is fragmentary. A sea with coral reefs and of
definite Middle Silurian age is known to have occupied the site of the
Rocky Mountains along the Alaska Highway. It appears to have been a
northern sea, connected with the Arctic Ocean across what is now Mac-
kenzie River Valley. Its southern and eastern extensions are not known,
and westward it may not have reached beyond the Trench. Armstrong
(1946) and Roots (1948) have recorded no Silurian or Devonian beds in
the Aiken Lake map-area, nor in any other area west of the Trench from
Omineca River to the Cariboo district. The region west of the Trench was
land (Armstrong, 1946) during Silurian and Devonian times, unless deposits
of these ages have been overlooked there or have been removed prior to the
Carboniferous period.

The Devonian history is only a little better known than the Silurian.
At times sedimentation was interrupted (See Chapter III), due, possibly,
to temporary emergence above the sea. During a part of Middle Devonian
time, a sea with coral reefs lay on the site of the northern Canadian Rockies,
and, like the mid-Silurian coral sea, may have been connected with the
Arctic Ocean across the site of Mackenzie River Valley. Like the Silurian
coral sea, also, it may not have extended west of the Trench. Later, during
part of Upper Devonian time, black muds accumulated in a sea that
extended northward across the site of Mackenzie River Valley and south-
ward an undetermined distance.

Mid-Palezozoic sedimentation was not disturbed by vulcanism. No
flows, tuffs, or breccias are interbedded with the sediments.

In contrast with the inferred restriction of the mid-Palwozoic seas
to areas east of the site of the Trench, the later Paleozoic seas appear to
have extended far to the west beyond the Trench, for sediments of the
late Palzeozoic, Cache Creek group have been recorded from the Aiken Lake
map-area by Roots (1948). The sea bottom of this time was not disturbed
by vulcanism east of the site of the Trench, but west of the Trench, Roots
(1948) records altered flows, tuffs, and agglomerates interbedded with the
sediments of the Cache Creek group.

The Palzozoic-Mesozoic contact and the nature of the latest Palseo-
zoic and earliest Mesozoic sediments have not yet been sufficiently studied
to infer all of what happened during the passage from one era into the other.
It was not marked by orogeny east: of the Trench ; the interruption of
marine aggradation was not prolonged there, for beds of Permian age are
preserved in places and fairly early Lower Triassic beds have been recog-
nized. Some uplift and erosion, of course, may have taken place.