Arkell.1956.Jurassic..

622 GENERAL SURVEY
whole area has been protected by the rigid foundations of a pre-Devonian
platform which lies not far beneath. Long but very gentle anticlinal
ripples alone break the monotonous plains, in which dips are hardly
perceptible.
The labile shelves are typified on the other hand by NW. Europe.
Here the Jurassic facies is similar and there are often also condensed
beds, but the formations tend to swell out into thicker lenses, there has
been considerable tectonic disturbance of a minor order (Stille's Germanotype
tectonics), and a number of ancient horsts break through the sedimentary
cover—such as the Black Forest, Bohemian Forest, Ardennes
Island and Scottish Highlands. There is evidence that in the Jurassic
most of these horsts were already upstanding or else began to rise. The
foundations of the labile shelves are the worn-down mountain chains
thrown up by the Variscan and Caledonian orogenies. Such a foundation
is a mosaic of structures and varying rock-types. Although solidified
and welded together by earlier orogenies, these tracts were still unstable
during the Mesozoic, and the nature of the foundations is sufficient
explanation of the contrast in their histories in comparison with the stable
shelves.
On the labile shelf of Europe there was in Jurassic times a complex
network of subsiding troughs of sedimentation of various shapes and sizes.
These grade imperceptibly into geosynclines (Arkell, 1933, p. 616), but
it is preferable not to stretch this term to cover them, for they seldom
contain more than 1000 m. of Jurassic sediments, they were not the seat
of volcanic activity or greywacke-type sedimentation, and they have not
been converted into folded mountains.
Troughs of subsidence and sedimentation on the labile shelves are
called cuvettes by Wills (1929, 1951), parageosynclines by Stille (but this
term was preoccupied in another sense by Schuchert: see Glaessner &
Teichert, 1947, p. 588), and intracratonal geosynclines by Kay (1951,
p. 107), who subdivides them into three types, each with a formidable
new name, according to shape, setting and filling. Here they will be
called by their English name, troughs, or troughs of deposition. The
nature, extent, subsidence and contemporaneous tectonics of the troughs,
with their cyclic sedimentation and its causes, have already been discussed
at length in relation to the British Jurassic, and all this need not be repeated
here (Arkell, 1933, chapter iii).
For data on some of the largest shelf regions in the Jurassic outside
Europe, the reader should refer to the Trans-Erythraean trough (p. 298)
and the Western Interior of the United States (p. 544). The American
example has been admirably monographed from this point of view by
Imlay (1949) without the use of any daunting terminology. It also
illustrates how a shelf trough can pass imperceptibly into a geosyncline.
The continental-lacustrine Jurassics of eastern Australia accumulated
in a broad trough which is tectonically analogous to the Rocky
Mountain trough of the Western Interior of the United States. The
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