FAUNAL SUCCESSION OF THE UPPER BERNICIAN 595 



the Great Scar Limestone series. The higher Yoredale strata 

 disappear as they are followed southward ; till in Wharfedale 

 the Millstone Grit comes almost immediately upon the Main 

 (= Great) Limestone. 



III. 



PHYSICAL CONDITIONS PREVAILING DURING THE 



DEPOSITION OF THE SERIES. 



This series of thick detrital accumulations, containing 

 numerous beds of coal and frequent interpolations of calcareous 

 strata, could only have been formed in the proximity of land, 

 and under very varying conditions, involving continuous oscil- 

 lation of the sea floor and repeated change in geographical 

 relationships. The rate of accumulation of sediment must at 

 one period have exceeded the rate of subsidence, so that the 

 sea became so silted up as to form a swamp in which grew the 

 jungles that formed the coal seams. Rapid submergence 

 followed, and these vegetable accumulations often became the 

 floor upon which the calcareous deposits were laid down. The 

 limestones were deposited near enough to the coast to allow 

 frequent invasions of fine sediment, as indicated by the numer- 

 ous shaly partings and interbedded shales. 



IV. 

 STRATIGRAPHICAL SUBDIVISIONS. 



The limestones of the Calcareous Division may be divided 

 . into three groups. The limestones above the Little Limestone, 

 which occupy the south and east of Northumberland and 

 which thin out towards the north, form the Upper Group ; the 

 Middle Group comprehends those thick and continuous beds 

 which can be followed right across the county, and includes 

 the Great Limestone ; whilst to the Lower Group belong the 

 limestones of the Calcareous Division below the Eelwell Lime- 

 stone. 



