e junction, we have the great series of deposits
which are known as the _Oolitic Rocks_, from the common occurrence
in them of oolitic limestones, or as the _Jurassic Rocks_, from
their being largely developed in the mountain-range of the Jura,
on the western borders of Switzerland. Sediments of this series
occupy extensive areas in Great Britain, on the continent of
Europe, and in India. In North America, limestones and marls
of this age have been detected in "the Black Hills, the Laramie
range, and other eastern ridges of the Rocky Mountains; also
over the Pacific slope, in the Uintah, Wahsatch, and Humboldt
Mountains, and in the Sierra Nevada" (Dana); but in these regions
their extent is still unknown, and their precise subdivisions
have not been determined. Strata belonging to the Jurassic period
are also known to occur in South America, in Australia, and in
the Arctic zone. When fully developed, the Jurassic series is
capable of subdivision into a number of minor groups, of which
some are clearly distinguished by their mineral characters, whilst
others are separated with equal certainty by the differences of
the fossils that they contain. It will be sufficient for our
present purpose, without entering into the more minute subdivisions
of the series, to give here a very brief and general account
of the main sub-groups of the Jurassic rocks, as developed in
Britain--the arrangement of the Jura-formation of the continent
of Europe agreeing in the main with that of England.

I. THE LIAS.--The base of the Jurassic series of Britain is formed
by the great calcareo-argillaceous deposit of the "Lias," which
usually rests conformably and almost inseparably upon the Rhaetic
beds (the so-called "White Lias"), and passes up, generally
conformably, into the calcareous sandstones of the Inferior Oolite.
The Lias is divisible into the three principal groups of the Lower,
Middle, and Upper Lias, as under, and these in turn contain many
well-marked "zones;" so that the Lias has some claims to be
considered as an independent formation, equivalent to all the
remaining Oolitic rocks. The _Lower Lias_ (_Terrain Sinemurien_ of
D'Orbigny) sometimes attains a thickness of as much as 600 feet,
and consists of a great series of bluish or greyish laminated
clays, alternating with thin bands of blue or grey limestone--the
whole, when seen in quarries or cliffs from a little distance,
assuming a characteristically striped and banded appearan