Arkell.1956.Jurassic..

PREFACE
The comprehensive Treatise of Geology has had its day. No man can
hope any longer to assimilate and summarize the whole known geological
history of the world. It has become almost a whole-time occupation to
keep track of the literature of a single geological system.
This book represents the first attempt at a synthesis of one system on
the basis of marine faunas in all parts of the world. Its object is twofold:
first, to present a unified description as a reference work, and second, to
enquire what can be deduced as to faunal realms, climate, palaeogeography,
volcanic and plutonic activity and earth-movements, and especially the
distribution of these in space and time, during one reasonably circumscribed
period of geological history. The prerequisite of all such investigations
is faunal correlation over the whole world, and no small part
of the object of this attempt has been to test the principles and performance
of palaeontological correlation on the world scale, to establish its
capabilities and limitations.
In the Jurassic system ammonites are by far the most important fossils
for correlation and consequently they are emphasized in this work, often
to the exclusion of other fossils. As originally planned, the book was to
have included a systematic review of Jurassic ammonite genera and
classification, but this has been detached for publication in the international
Treatise of Paleontology, edited by Professor R. C. Moore. The ammonite
volume of the Treatise and the present book are therefore to some extent
complementary.
The emphasis upon ammonites, regarded by the author as essential,
may seem to specialists in other branches of palaeontology to give a lopsided
picture. A review of all the different kinds of life in the Jurassic,
from plants and foraminifera to reptiles and mammals, might lend a
greater air of completeness and perhaps of balance; but it would greatly
increase the length and cost and would add nothing that cannot be found
in elementary text-books of geology and palaeontology. Instead, space
has been devoted to bringing together evidence on matters less well
known, such as faunal realms, permanence of oceans and the chronology
of tectonics.
In order to keep as nearly as possible to facts and avoid repeating
dubious or erroneous concepts, few palaeogeographical maps are provided.
When all the stratigraphical data are sifted it is surprising how often modern
information contradicts palaeogeographical maps that have been repeated
in book after book for decades. Originally such maps were stimulating
and the defence often heard is that they are fertile and suggestive, but it
seems to me that we have reached the stage when they often act as an
opiate and may do more harm than good. For instance, I am convinced
Vii
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