536 HISTORICAL PALEONTOLOGY. 



III. Middle Oolites (Oxford Clay and Coral-rag). 



IV. Upper Oolites (Kimmeridge Clay, Portland Stone, and 



Purbeck beds). 



I. The Lias succeeds the uppermost beds of the Trias with 

 perfect conformity, and passes upward, generally conformably, 

 into the lowest beds of the Lower Oolites. It consists essen- 

 tially of a great series of bluish or greyish laminated clays, 

 alternating with thin bands of blue or grey limestone, the 

 whole assuming at a distance a characteristically striped and 

 banded appearance. 



II. The Lower Oolites consist of calcareous freestones (In- 

 ferior Oolite), shales, clays, and marls (Fuller's earth), fine- 

 grained Oolitic limestones (Great Oolite), with calcareous 

 flags at the base (Stonesfield slate), and superiorly shelly lime- 

 stones and calcareous sandstones (Forest - marble and Corn- 

 brash), the whole having a thickness of from 400 to 500 feet. 

 In Yorkshire the Lower Oolites consist of limestones with car- 

 bonaceous shales and thin seams of coal, which are sufficiently 

 extensive and constant to be worked for coal. Of this age, 

 also, is probably the coal-field of Brora, in Sutherlandshire, in 

 the north of Scotland. 



III. The Middle Oolites are composed of a great mass of 

 dark-blue tenacious clay (Oxford clay), with a maximum thick- 

 ness of 700 feet, surmounted by from 150 to 250 feet of lime- 

 stones, known as the Coral-rag, from the number of corals con- 

 tained in them. 



IV. The Upper Oolites consist in Britain of laminated, some- 

 times carbonaceous or bituminous clays (Kimmeridge clay), 

 forming the base of the group, and having a thickness of 500 

 or 600 feet. These are succeeded by sandstones and lime- 

 stones (Portland stone) of about 120 feet in thickness; and 

 the formation is capped by a remarkable group of alternating 

 strata of fresh-water, brackish-water, and marine beds, with old 

 land-surfaces, the whole known as the Purbeck beds, and 

 having a united thickness of about 150 feet. Of the same age 

 as the Upper Oolites in Britain is the Solenhofen slate of 

 Bavaria, an exceedingly fine-grained stone, which is largely 

 used in lithography, and is celebrated for the number and 

 beauty of its organic remains, especially those of Vertebrates. 



Rocks belonging to the Jurassic series, in the form of lime- 

 stones and marls, have been detected by their fossils in the 

 Laramie Mountains and in other portions of the Rocky Moun- 

 tains, and also at various points in Arctic America. The ex- 

 tent, however, of these beds is unknown, and no subdivisions 

 have hitherto been established in them. 



