Arkell.1956.Jurassic..

534 CORDILLERAS OF NORTH AMERICA
All the outcrops being in the west, most of the faunas are recent discoveries,
and it is worth reflecting that hardly any of them were known
when Hyatt was working on ammonites at Harvard 60-90 years ago. In
those days it was easier and preferable to visit the museums of Europe
for material than to search the Rockies.
SOUTHERN ALASKA
The most complete sequence of Jurassic rocks on the North American
continent occurs in the coast ranges of southern Alaska, which sweep
in from the half-submerged Aleutian arc, through Alaska Peninsula and
Cook Inlet, through the Alaska Range, and then follow round, parallel
to the Pacific coast, to merge into the coast ranges of Canada. Their
outcrops, now dissected by severe glacial erosion, by inlets of the sea and
by intrusive batholiths, mark the site of a major geosyncline, comparable
in scale with those of eastern Asia and the rest of the Pacific seaboard of
the Americas. The total thickness of Jurassic sediments amounts to
about 4500 m. (Fig. 85, p. 537.)
The Lower Jurassic is mainly in volcanic facies, consisting of submarine
lavas, tuffs and agglomerates, passing into and interbedded with shales,
sandstones and conglomerates, in which in some districts Lower and
Upper Liassic marine faunas have been found. The Middle and Upper
Jurassic comprise a vast and varying pile of conglomerates, sandstones
and shales, with rich ammonite faunas occurring at several horizons.
Researches are not yet sufficiently far advanced to admit of satisfactory
generalization. Much detail is known of a number of scattered areas,
but in between are still greater areas as yet unexplored stratigraphically
or of necessity treated still as 'undifferentiated Mesozoics'. Three excellent
syntheses have been published (Martin, 1926; Smith, 1939; Imlay,
1952), and no attempt is here made to summarize once more the mass of
detail marshalled in these works and in the numerous Geological Survey
bulletins to which they refer.
The south Alaskan ammonites show close connexions with those of
the Arctic slope, Greenland, and the Arctic Ocean generally, but in the
absence of any connecting outcrops across the north-western shoulder
of the continent it cannot be stated definitely whether a seaway passed this
way or through the Bering Straits and over NE. Asia (cf. Crickmay, 1931,
maps 4-11; Eardley, 1951, p. 517). The latter view implies an exceptional
degree of disharmony between the Jurassic geosyncline and the strike of
later foldings: for it seems to be generally agreed that the Rocky Mountain
structures carry on NW. and then west into the Brooks Range of
northern Alaska, and account, presumably, for the tightly-folded and
semi-metamorphosed Jurassic of the Firth River region (p. 529). The
morphic continuation of the cordilleran geanticline is assumed to be the
basin of the Yukon River in central Alaska and its surrounding Palaeozoic
mountains, where the Mesozoics are largely absent or represented only by
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