Arkell.1956.Jurassic..

6i8
GENERAL SURVEY
the air; this may explain the contrast between the giant marine Portland
and the dwarf Purbeck freshwater shells.
If there was a general warming of the earth's surface by increased
radiation received from the sun, there would be nothing to show where
the poles were situated. Provided the general temperature was such
that no ice caps were formed, geophysicists are at liberty to postulate
almost any wanderings of the poles that may be convenient. They may
do this for most of geological history, for ice ages seem to have been
infrequent events; and their geological remains are so awkwardly arranged
that some wandering of the poles may be welcome as an aid to accounting
for their distribution.
The infrequency of glacial episodes and especially the rarity of fossil
tills in Arctic regions indicate that if, in fact, the poles have always been
approximately where they are now, the warm state of the earth in the
Jurassic was normal and our present condition, with polar ice caps, is
exceptional. In that case it rests with the astronomers to decide whether
the output of radiation from the sun has fluctuated enough and at the
required times, or whether part of the radiation has been intercepted on
occasion by interstellar dust or gas.
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