136 PROCBEDrNGS OF THE GEOLOGICAL SOCIETY. [Feb. 5, 



are either partially or altogether denuded, estimated upon certain 

 definite principles. Of such principles the development of calcareous 

 and " sedimentary " strata from opposite directions is one of the 

 most important. 



It will be perceived that isometric lines may be used in repre- 

 senting the thickness of an individual stratum, as well as of forma- 

 tions, groups, or systems ; and the chief point to be attended to in 

 tracing them is that the calcareous elements be eliminated from the 

 " sedimentary." 



In the case of " sedimentary " strata, a series of isometric lines, 

 each representing an equal increase or diminution in thickness, will 

 become nearer or wider apart as they approach or recede from the 

 centre of maximum development. 



In the case of calcareous formations, the focus or centre of maxi- 

 mum development will be at opposite points to that of the " sedi- 

 mentary " in the same group or system of rocks, and the isometric 

 curves will intersect, gradually diminishing in force from their re- 

 spective centres, just as a series of waves propagated from two centres 

 of disturbance cross each other and gradually die away in opposite 

 directions. (See Map, PL YII.) 



§ II. Carboniferous Land-surface of Central England. — Having 

 thus explained the nature of isometric lines, we proceed to consider 

 their apphcation to the Carboniferous Rocks of Britain. I believe, 

 they will be found of essential service in bringing clearly and intel- 

 ligibly before the eye several phenomena connected with the distri- 

 bution of the sedimentary as compared with the calcareous portions 

 of this group. 



It is necessary that a few words should be said in reference to a 

 point of interest in the physical geology of our island, which should 

 be clearly understood before treating of the distribution of the Car- 

 boniferous strata. I refer to the existence of a barrier of land 

 which there are grounds for supposing to have stretched from Wales 

 eastward, skirting the southern ends of the South Staffordshire and 

 Warwickshire Coal-fields, and including the Cambrian Rocks of 

 Chamwood Forest. The evidences for the existence of this land- 

 surface I cannot here stop to point out in detail, having already done 

 so elsewhere * ; suffice it to say that they are numerous and satis- 

 factory, both on general physical grounds and from phenomena ob- 

 served in the mines of the coal-fields on approaching its borders. 

 This barrier (which possibly was an extension of the Scandinavian 

 promontory on the one hand, as very clearly indicated by Mr. God- 

 win-Austen, and thence stretched across the Irish Sea to embrace 

 the Cambro- Silurian districts of Wicklow and Carlow on the other) 

 divided the Carboniferous Rocks of South Wales, Somersetshire, and 

 Dean Forest from the coal-tracts of Central and Northern England 

 and Scotland (see Map) ; and, as we shall see, the strata on each 

 side belong to two distinct systems of distribution, and are due to 

 two different sets of oceanic currents. 



* ' The Coal-fields of Great Britain,' 2nd edit. p. 246 et seq. 



