1859.] HULL THINXING-OUT OF THE SECONDARY ROCKS. 67 



tainty define the southern limit of the formation under the districts 

 of Oxfordshire and Northamptonshire ; but, recollecting the absence of 

 these rocks below the Chalk near London, Boulogne, and the Franco- 

 Belgian Coal-field, it is probable that they become attenuated south- 

 wardly from Warwickshire. From considerations founded on current- 

 structures, Mr. Sorby seems to have arrived at a similar conclusion. 



§ 3. South-Easterly Attenuation of the Lower Secondary Roclcs. 



1. The Trias. — A. Bunter Sandstone. — Our inquiries commence 

 with this formation, which introduces the Mesozoic Series, and 

 rests unconformably on all the rocks of Palaeozoic age. Attaining 

 its highest development towards the north-west, it composes the 

 plains of Cheshire and Lancashire ; it fills the beds of old Palaeozoic 

 valleys, as those of the Eden, the Clewyd, and Belfast Lough ; and 

 from its position along the sea-coasts of Ireland, the Isle of Man, 

 North Wales, Lancashire, and Cumberland, it probably forms a large 

 portion of the bed of the Irish Sea. 



It is, however, only recently, since the detailed examination 

 carried out by the officers of the Geological Survey, that anything 

 like a just estimate of the stratigraphical importance of this forma- 

 tion has been arrived at. It has been classified into three subdi- 

 visions, sufficiently marked to enable us to compute the depth of the 

 formation in any given district, and such as allows us in the present 

 inquiry to arrive at definite results. 



From a section across the plain of Chester, and Delamere Forest, 

 we obtain the following vertical measurement of these subdivisions in 

 this district*. 



Hunter Sandstone. Cheshire. North-west District. 



'■'>. Upper mottled Sandstone 700 ft. 



2. Conglomerate-beds 750 



1 . Lower mottled Sandstone 7< H » 



Total thickness 2150 



The general succession of the Trias in North-west Cheshire i* 

 shown in PI. IV. fig. 2. 



A fow miles further south, at Holt, another section gives the 

 thickness of the Bunter Sandstone at 1600 feet; and a third, across 

 Bridgcnorth, in Salop, shows nearly the same result. 



Now, whenever we follow these beds eastward, from their furthest 

 western outcrop, we invariably find that tiny decrease in thickness ; 



Some of the Subdivisions more rapidly than others. In the counties 



of Derby, Stafford, Leicester, and Warwick, the l >t and 3rd sub- 

 divisions are generally absent, OT only sparingly represented. The 

 Conglomerate-beds predominate frequently where the other sub- 

 divisions have disappeared ; bul when wo cross to the eastern 



lionlers of the Leicestershire and Warwickshire Coal -field-, this most 



constant subdivision thins away. We find the Lower Reaper 



Sandstone resting directly on the Coal-measures nt Ashby, and 



• Eoriion. Beet of tlio Geol. Survey, Sheet 43. 



F 2 



