1865.] Hull on the History of the British Coal Measures. 25 



thickness from the north-west towards the south-east of England, so 

 that a series of lines, each representing a certain thickness of beds 

 wherever drawn, would be found to cross the country obliquely from 

 south-west to north-east.* The Trias attains its greatest vertical 

 development in Lancashire, and from this gradually thins away towards 

 Warwickshire ; so that while in the former county the thickness may be 

 placed at nearly 5,000 feet, in the latter it is only 600 where it passes 

 below the Lias. Here we lose all sight of it, but judging from analogy, 

 we may conclude that it thins away altogether somewhere about the 

 line of the chalk escarpment ; and we know from actual experiments 

 that it does not reach the sea at Harwich. 



By a series of similar comparisons we find that the Lias undergoes 

 a similar change in thickness, dying off towards the south-east ; and 

 there is reason to believe that the succeeding clayey beds of the 

 oolitic groups prove no exception to the rule. Now as these forma- 

 tions, owing to denudation,| terminate abruptly in the direction of 

 their maximum of thickness, in other words, towards the north-west, 

 we may regard them as a series of great wedges lying over each other 

 in succession, with their thin edges directed towards the south-east 

 coast, but never actually reaching so far. 



The view here adopted of the close proximity of the ancient Cam- 

 bro-Silurian rocks beneath the Cretaceous in the south-east of England 

 receives confirmation from certain characteristics of this latter group 

 itself. Conglomerates, consisting of black hornstone, slate, and quartz, 

 are not uncommon ; and in the notable sinking made in search of 

 water at Kentish Town, near London, a conglomerate of syenite, 

 greenstone, porphyry, quartz, and schistose pebbles was reached at a 

 depth of 1,122 feet from the surface. These pebbles being the detritus 

 of subaerial rocks in the neighbourhood of the strata which were 

 being formed over the bed of the sea at the commencement of the 

 Cretaceous period, seem to indicate the total absence of the softer 

 strata of the later Pakeozoic and earlier Mesozoic periods. 



4. Tlie Mutual Relationship of these Formations, and their Teachings. 



From the above considerations it will be apparent that w r hile the 

 coal-formation attained its greatest development in the north of 

 England, the formations which overlie it also attained their greatest 

 vertical dimensions in nearly the same direction ; and since the coal- 

 measures are brought to the surface in Lancashire, Staffordshire, and 

 North Wales, it follows that the elevating forces and the agencies of 

 denudation have acted with greatest effect over these parts, that is to 

 say — looking at the beneficial results — where most needed. How im- 

 penetrable would have been the covering which once overspread the 

 coal-measures of the north-west of England may be judged by estimat- 

 ing the thickness of the strata which we may infer formerly covered 



* Such lines I have traced for the Carboniferous group under the term " isome- 

 tric lines.'' Bee ' Quart. Journ. Geol. Soc.,' vol. xviii. p. 127. 



t "Denudation" is a term used to express the sweeping away by the sea, 

 rivers, &<\, of portions of the strata. 



