Arkell.1956.Jurassic..

CHAPTER I
CLASSIFICATION AND CORRELATION
THE JURASSIC AMONG THE SYSTEMS
As the middle system of the middle (Mesozoic) group of fossiliferous
rocks, the Jurassic is as good a sample as any that could be chosen to find
out what light can be thrown on geological problems by study of a single
system all over the world.
In the popular imagination the Jurassic is the period of great marine
reptiles and flying dragons. To the average palaeontologist it is perhaps
primarily the period of the diminutive proto-mammals and toothed birds,
of the first known fully-elaborated freshwater molluscan faunules, and of a
bewildering host of ammonites. To the palaeobotanist it is a period of
somewhat monotonous world-wide floras composed mainly of conifers,
ferns and cycads, but also of the first possible indications of angiosperms.
To the palaeoclimatologist it is characterized by uniformity of climate
and lack of evidence for glaciation. The igneous petrologist and volcanologist
find relatively little that is not displayed much better in other systems,
and to the structural geologist the Jurassic was the most passive of the
geological periods.
For the stratigrapher, however, the Jurassic is the very well and fountain
of his subject. It was on the Jurassic rocks that William Smith, Father
of Historical Geology, founded the science of stratigraphy, enunciated the
law of superposition, identified fossils with particular strata, and named
the classic formations. It was on Jurassic rocks that Oppel founded
modern zonal stratigraphy and named the classic zones. It was for
Jurassic rocks that d'Orbigny introduced the first scheme of stages. All
these concepts became part of the fabric of stratigraphical geology the
world over.
Although the approximately 25 million years of the Jurassic period
(from about 152 to 127 million years ago according to Holmes, 1947)
were on the whole tectonically quiet, they nevertheless witnessed extensive
changes. It was a period of preparation for the mountain-building
revolutions to come—for the most part in the Cretaceous or Tertiary, but
in North America, the Caucasus and Crimea shortly before the end of
the Jurassic. The preparations consisted of long-continued subsidence
of geosynclines and the filling of them with sediments, several thousands
of metres thick, which implies massive denudation of uplands. There
were also more widespread, less localized, subsidences and upheavals, of
epeirogenic type. These were less spectacular than the orogenic and
pre-orogenic movements but they were more important for organic
evolution. By opening and closing seaways, controlling ocean currents
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