226 HISTORICAL PALZONTOLOGY. 
CHAPIRE Xvi. 
THE: FURASSIC. PERIOD: 
Resting upon the Trias, with perfect conformity, and with 
an almost undeterminable junction, we have the great series of 
deposits which are known as the Oolztte Locks, from the com- 
mon occurrence in them of oolitic limestones, or as the /wras- 
sic 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 Ne- 
vada” (Dana); but in these regions their extent is still un- 
known, and their precise subdivisions have not been deter- 
mined. 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. Tue Lias.—The base of the Jurassic series of Britain 
is formed by the great calcareo-argillaceous deposit of the 
“Tjas,” which usually rests conformably and almost inseparably 
upon the Rhetic beds (the so-called “ White Lias”), and. 
passes up, generally conformably, into the calcareous sand- 
stones 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 inde- 
pendent formation, equivalent to all the remaining Oolitic 
rocks. The Lower Lias (Terrain Sinemurten of 1)’Orbigny) 
sometimes attains a thickness of as much as 600 feet, and con- 
sists of a great series of bluish or greyish laminated clays, 
