Arkell.1956.Jurassic..

THE CAUCASUS 357
immense thicknesses, in other places emersion, isoclinal folding, subaerial
volcanism and outbreaks of lava from submarine fissures, all during
Jurassic times. In attempting to synthesize such a region there is a choice
between a catalogue of observations, in which the reader would become
lost in a mass of detail and a maze of outlandish names, and a brief summary
of the area as a whole. The second choice is rendered inevitable both by
reason of space and by the fact that observations backed by modern work
on the ammonites are still so scattered, and separated by so many gaps,
that nothing like completeness could yet be achieved even were all the
known details presented.
In broad outline the structure of the Caucasus is an anticlinorium.
In the western half the Jurassic formations are pierced by an elongated
central core of crystalline rocks, igneous and metamorphic, partly Palaeozoic
but mainly older, rising in the central Caucasus for many miles above
10,000 ft. Upon it stand the late-Pleistocene to Recent volcanoes of Elburs
(18,523 ft.) and Kazbek (16,546 ft.). In general the Jurassic sediments
dip off the flanks of this watershed in parallel ranges falling northwards
in the Kuban and southwards in Georgia. The eastern half consists
entirely of Jurassic rocks (perhaps older in the centre) which in Daghestan
form a series of parallel ranges in the south limb of the main anticline, and
a great plateau, cut by gorges of north-flowing rivers, in the north limb.
In general, however, the outcrops trace a series of concentric ellipses,
the youngest formations on the outside, the oldest in the centre. The
central Main Ridge of Daghestan consists of isoclinally folded metamorphic
slates, phyllites, quartzites, etc., of which the age is still questionable
: they may be Lower Liassic, Triassic, or even in part Palaeozoic.
The unmetamorphosed sedimentary series of proved Liassic age is
immensely thick (8500 m. for the Toarcian and Lower Bajocian alone)
and in places rests directly on pre-Cambrian gneisses with a basal conglomerate;
and even the Bajocian contains granitic debris. Right up to
the Upper Bajocian the rocks are coal-bearing, with in some places only
minor marine intercalations. On these grounds it has generally been
supposed that the crystalline core was an island in Jurassic times, but long
strips of Liassic sediments faulted into the midst of it, shown on the
Russian Geological Survey map of 1929, make this interpretation extremely
doubtful and suggest that the plant-remains are drifted. The elastics
appear to have been derived from the north (Mokrinskij, 1939).
A fully marine regime became established in some places in the Pliensbachian
and probably everywhere during the Bajocian. The subsidence
was accompanied in the Middle Bajocian by volcanism, which produced
great thicknesses of tuff in the sediments of the southern slopes of the
western Caucasus and in the outcrops of the Little Caucasus, to the
south of the Kura River basin. At the same time there was a massive
outflow of submarine lava. This series in Georgia is 2000 or even 3000 m.
thick (Mokrinskij, 1939, p. 509). The scene of maximum volcanicity
appears to have been the north-eastern Little Caucasus. Here the
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