internal Structure of the Magnesian Limestone. 41 



The escarpment at the western limit of the formation is seldom of simple 

 structure, but is generally composed of beds of sand, sandstone, or coal shale, 

 capped with beds of magnesian limestone of variable thickness. That the 

 whole region which is traversed by this limestone has been ravaged by powerful 

 denuding forces, is proved by the contour of the neighbouring districts, by the 

 accumulations of diluvial rubbish, by the valleys of denudation, by the deep 

 bays which have been scooped out of the escarpment, and, above all, by the 

 outliers which in some places are found considerably to the west of the 

 general boundary of the formation. It is, I think, plainly demonstrated by 

 the configuration of the country, that some of the upper portions of the coal 

 formations of Nottinghamshire, Derbyshire, and Yorkshire, have been saved 

 from destruction by the capping of the limestone. At the same time the pre- 

 sent drainage of these counties shows that the escarpment above mentioned 

 opposed but a feeble barrier against the great denuding torrents which de- 

 scended from the western chain to the plain of the new red sandstone ; for 

 the rivers which have their source in the higher regions to the west of the 

 magnesian limestone are not, on reaching its escarpment, deflected from their 

 course, but generally cut through it by a direct channel into the great central 

 plain of England : and, near the northern extremity of Yorkshire, where the 

 limestone abuts against a mountainous tract of country (without the interven- 

 tion of any lower region of the coal strata), the descending currents have 

 swept away large portions of the formation, which can only be traced through 

 that district by a few inconsiderable fragments which are still found in situ, 

 and indicate the general line of its original direction*. The drainage of the 

 county of Durham might at first sight be considered an exception to the pre- 

 ceding remarks ; for, after descending from the mountains, the river Wear, 

 during a considerable part of its course, runs nearly parallel to the range of 

 the magnesian limestone. The exception is, however, only apparent; for the 

 waters descend in a natural depression of the great coal basin, and are not at 

 all deflected by the magnesian escarpment. 



* Tho terrace of the great oolite, on the contrary, produces in almost every part of England a 

 striking effect upon the direction of the rivers, as will be seen by a glance of the eye over the 

 geological map. This fact may be accounted for by three causes. 1st. The oolitic terrace is of 

 much greater magnitude than the magnesian terrace. 2dly. The oolitic terrace is composed of 

 more coherent materials, and is less intersected by fractures and dislocations. 3rdly. This terrace 

 in the greatest part of its range is separated from the higher western regions by the great plain of 

 the new rod sandstone. The denuding torrents may therefore have spent a part of their fury in 

 the central plain of our island, before they reached the base of the great oolitic escarpment. All 

 these causes may have combined in preventing some of the ancient denuding currents from forcing 

 their way through the escarpment of the great oolite. 

 VOL. III. SECOND SERIES. G 



