South-western Coal District of England. 2\3 



vestig-ating the subterranean geology of the incHned beds of the district^ from 

 the mere occurrence at the surface of the rock which is the lowest in the 

 overlying series^ to augur respecting the inclined stratum that shall be found 

 beneath the overlying one. Those persons must therefore be in error who, 

 from an existing superstratum of red marl, and from that occurrence only, are 

 led to infer the existence of coal-measures lying beneath it*. 



On the Mode of Distribution and Local Extent of the Rock Formations of the 



First Series. 



From extensive observations made on the dip of the inclined strata through- 

 out the district, we find that they are subject to undulations, which dispose 

 them into a number of basins of less or greater extent, according as those un- 

 dulations are more or less rapid, the strata dipping from every point in the cir- 

 cumference of each basin towards its centre. 



These basins are divided from one another by lines, which may be termed 

 anticlinal lines, formed by the saddles of the strata, or meetings at the surface 

 of their vertical angles ; on either side of which the strata dip in opposite di- 

 rections. For rendering the structure of such a country, as relates to the in- 

 clined strata, intelligible, it is of the first importance to determine the position 

 of these lines, and thus resolve the district into its component basins. We 

 have therefore endeavoured to represent the course of these lines in the accom- 

 panying Map. 



Sometimes these lines follow the crests of the chains of hills (see Fig. 1); 

 sometimes they are to be traced along the course of valleys (see Fig. 2) : the 

 hills in the latter case being formed, not by the saddles of the strata, but by 

 the escarpments cutting through them. 



Fig. 1. Fig. 2. 



To apply these principles to the district before us. — The grey-wacke and 

 the transition limestone appearing in but few places, the old red sandstone 

 may be considered as the fundamental rock of the district. The undulations 

 of the fundamental rock will determine those of the super-strafa of the same 

 series ; since the upper and lower strata are parallel with regard to each other. 



* See Mr. Warburton's " Observations on the overlying strata in the neighbourhood of Bris- 

 tol," printed in the 4th volume of the Geological Transactions, 1st series. 



