THE JURASSIC PERIOD. 233 



deposits which are known as the Oolitic Rocks, from the com- 

 mon occurrence in them of oolitic limestones, or as the Juras- 

 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 Austrafia, 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 sand- 

 stones of the Inferior Oolite. The Lias is divisible into the 

 three principal groups of the Lower, Middle, and Upper Lias, 

 as below, 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 Sinemurien of D'Orbigny) 

 sometimes attains a thickness of as much as 600 feet, and con- 

 sists 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 appearance. 

 By means of particular species of Ammonites, taken along with 

 other fossils which are confined to particular zones, the Lower 

 Lias may be subdivided into several well-marked horizons. 



