CHAP. IX.] JURASSIC PERIOD. 157 



that even the Scottish coast-lines were carried back so far 

 that the same clay was spread over areas where only estua- 

 rine deposits had previously been laid down (see p. 143) ; 

 but whether there was continuous sea outside the east of 

 Scotland, as Professor Hull thinks, is quite uncertain. 

 In this great deposit of dark blue clay we seem to have a 

 repetition of Liassic conditions, and it is highly probable 

 that the source of supply was the same namely, the Coal- 

 measures which once covered so large a portion of Wales, 

 Ireland, and North Britain. 



The episode of the Corallian Beds marks a time when 

 from some cause the deposition of mud ceased over certain 

 parts of the sea-bottom, and the water became clear enough 

 for the growth of coral-reefs with their accompaniments of 

 calcareous sands, marls, and oolitic limestones. This episode 

 may have been caused either by the temporary diversion 

 of the mud-bearing currents, or by the diminution of the 

 supply, in the manner already explained when treating of 

 the change from Lias to Oolites. The latter is, perhaps, 

 the most probable cause, and we may regard the Corallian 

 stage as indicating a pause in the movement of subsidence 

 during which less muddy material was carried down by the 

 rivers. The succession of beds in Sutherland lends some 

 confirmation to this view ; there as elsewhere the Oxf ordian 

 clays and shales tell of submergence, but the overlying 

 sandstones (see p. 143) show that the currents were able 

 to carry sand further out from the shore, and over the deep 

 sea mud, as they might do if the land was stationary for 

 a time, and just as an extension of sand over mud is be- 

 lieved to be now taking place in the English Channel. 1 

 In the succeeding series of alternating marine and estuarine 

 beds we trace the progress of a further subsidence. It 

 would appear, therefore, that the intercalation of white 

 sandstones in the shallow waters of the Scottish area, and 

 1 See Godwin-Austen, " Quart. Journ. Geol. Soc.," rol. Ti. p. 82. 





