1865.] Hull on the History of the British Coal Measures. 23 



at a distance from the source of supply ; and. thirdly, the velocity of 

 the current may be great or small, the transporting power of which is 

 as the sixth power of the velocity. Observations on the accumulated 

 thickness of strata or groups are instructive, as tending to throw light 

 on these questions, but the subject regarded in this light has not 

 received its due share of attention. Classifying the strata which com- 

 pose the Carboniferous group into Calcareous and Sedimentary,* and 

 confining our attention to the latter, we can compare the relative 

 thickness of these beds with their representatives in various parts of 

 the country, and in so doing arrive at some very interesting results, 

 for we find the thickness changing according to a definite plan or 

 arrangement. On the north side of The Barrier we find a constant 

 diminution in the vertical thickness of the group on proceeding from 

 north to south, while on the south side of The Barrier the decrease 

 takes place from west to east, showing a change of physical conditions, 

 and the marked influence which this dividing ridge has exerted upon 

 the distribution of the sedimentary materials. f Supposing the velocity 

 of the currents which have transported the sand and clays of the coal- 

 measures to have been the same on both sides of The Barrier, we may 

 conclude that the thinning of the beds arises from the two first causes 

 above stated, namely, the approach to a shelving shore and the dimin- 

 ishing supply of sediment as the distance from the sources of supply 

 increases. It would therefore appear that north of The Barrier the 

 source of supply lay to the north ; and south of The Barrier the 

 source lay to the westward. These sources must have been lands 

 traversed by rivers, bringing down sand and mud which the currents 

 of the sea took up and distributed over the ocean bed. J 



As illustrations of the thinning away of the beds on either side of 

 The Barrier from north to south in one case, and from west to east in 

 the other, let us take the following from districts lying in nearly 

 direct lines in each case, which have been very carefully measured 

 during the progress of the Geological Survey. 





North of the 



Barrier. 









Lancashire. 

 Feet. 



North Staffordshire. 

 Feet. 



Leicestershire. 

 Feet. 



Upper Coal-measures 

 Middle „ 

 Lower ,, 

 MiUstone Grit 

 Yoredale Beds 



. 2,000 



. 3,500 



. 1,800 

 . 3,500 

 . 2,000 



1,000) 

 4,000 ( 

 1,000 

 500 

 2,300 



2,000 



1,000 

 50 

 50 



12,800 8,800 3,100 



* That is, limestones on the one hand, and sandstones, clays, shales, &c, on the 

 other. 



t I take for granted that all my readers admit that the sedimentary strata have 

 heen deposited from sands, mud, &c, held in suspension by the sea and carried by 

 currents. 



X The depth of shading on the map will indicate what may be considered the 

 deep and shallow parts of the submerged areas. 



