CHAPTER VII. 



DYASSIC OR PERMIAN PERIOD. 



1. Stratigraphical Evidence. 



T7XCEPT in the north-east of England the Dyassic 

 IJ rocks occur only in fragmentary strips and patches 

 along the borders of the Coal-measure basins. This dis- 

 connected mode of occurrence is partly due to the over- 

 stepping of the Trias, under which many areas of Dyas are 

 buried and concealed, and partly of course to the removal 

 of large portions by erosion. Their limitation, however, to 

 certain districts and their entire absence in the south of 

 England are facts due to the conditions under which they 

 were originally deposited. The Dyassic beds exhibit two 

 distinct lithological f acies, which may be called the eastern 

 and western types; the latter being the more local and 

 abnormal, while the former is similar to that which pre- 

 vails in Germany. 



A. The rocks of the eastern type are supposed to under- 

 lie the greater part of East Yorkshire, Lincoln, and Not- 

 tingham, and their outcrop forms a continuous strip of 

 ground, between the Carboniferous and Triassic strata, 

 from the coast of Durham, southward by Auckland, Ripon, 

 and Pontefract, to the neighbourhood of Nottingham. 



The beds are thickest and most purely calcareous in 

 Durham, where they consist almost entirely of dolomite 

 or magnesian limestone. The basement beds are soft sand 

 and calcareous shale of variable thickness ; the limestones 



