cur in the upper
calcareous grit at North Grimstone; and in the middle and lower
calcareous grits good building stones are found.

Among the fossils in the English Corallian rocks corals play an
important part, frequently forming large calcareous masses or "doggers";
_Thamnastrea_, _Thecosmilia_ and _Isastrea_ are prominent genera.
Ammonites and belemnites are abundant and gasteropods are very common
(_Nerinea_, _Chemnitzia_, _Bourgetia_, &c.). _Trigonias_ are very
numerous in certain beds (_T. perlata_ and _T. mariani_). _Astarte
ovata_, _Lucina aliena_ and other pelecypods are also abundant. The
echinoderms _Echinobrissus scutatus_ and _Cidaris florigemma_ are
characteristic of these beds.

Rocks of the same age as the English Corallian are widely spread over
Europe, but owing to the absence of clearly-marked stratigraphical and
palaeontological boundaries, the nomenclature has become greatly
involved, and there is now a tendency amongst continental geologists to
omit the term Corallian altogether. According to A. de Lapparent's
classification the English Corallian rocks are represented by the
_Sequanien_ stage, with two substages, an upper _Astartien_ and lower
_Rauracien_; but this does not include the whole Corallian stage as
defined above, the lower part being placed by the French author in his
_Oxfordien_ stage. For the table showing the relative position of these
stages see the article JURASSIC.

See also "The Jurassic Rocks of Great Britain," vol. i. (1892) and
vol. v. (1895) (_Memoirs of the Geological Survey_); Blake and
Huddleston, "On the Corallian Rocks of England," _Q.J.G.S._ vol.
xxxiii. (1877). (J. A. H.)

CORAL-REEFS. Many species of coral (q.v.) are widely distributed, and
are found at all depths both in warmer and colder seas. _Lophohelia
prolifera_ and _Dendrophyllia ramea_ form dense beds at a depth of from
100 to 200 fathoms off the coasts of Norway, Scotland and Portugal, and
the "Challenger" and other deep-sea dredging expeditions have brought up
corals from great depths in the Pacific and Atlantic oceans. But the
larger number of species, particularly the more massive kinds, occur
only in tropical seas in shallow waters, whose mean temperature does not
fall below 68 deg. Fahr., and they do not flourish unless the
temperature is considerably higher. These conditions of temperature are
found in a belt of ocean which may roughly be indicated as lying between
the 28th N. and