358 Prof. Sedgwick on the Classification of the Strata [MaV, 



the remaining part of the coast as far as Saltburn, it forms a 

 succession of magnificent escarpments, the greatest number of 

 which are, however, crowned by the lowest beds of the superior 

 formation. Lastly, after being concealed for three or four miles 

 by a low cliff of blown sand, it reappears in the reef which runs 

 out into the sea near Redcar. 



It would, perhaps, be impossible to give correctly the range 

 of the western boundary of the alum-shale through the interior 

 of the district. From its appearance at Redcar, as well as in the 

 quarries opened in the trap dyke near Langbargh, it is evident 

 that the line passes through the plain at some distance to the 

 west of the great escarpment of the Cleveland hills. In every 

 part of this escarpment, the shale rises to a great elevation ; and 

 by its gradual dip towards the south-east is at length carried 

 under the north-western skirt of the Hambleton hills, and disap- 

 pears altogether, the western boundary being probably buried 

 under the diluvium of the plain. 



The greatest part of the Yorkshire moorlands north of the ool- 

 ite hills is composed of the different members of the coal forma- 

 tion described in the preceding section.* The alum-shale is, 

 however, laid bare in the deep valley of the Esk, and in some 

 other similar situations, at a considerable distance from the coast. 



For an admirable detailed account of this formation, the 

 reader may consult the " Survey of the Yorkshire Coast," p. 

 127 — 160. A very short account of it will be sufficient for my 

 present purpose. The beds which appear on the coast may be 

 conveniently divided into the following groups, beginning with 

 the highest. 



(1.) A group, nearly 250 feet thick, composed as follows: 

 — 1. Soft glossy unctuous slate clay, 100 feet thick, abound- 

 ing with pyritous nodules, and extremely liable to spon- 

 taneous decomposition. It is from this part exclusively that 

 alum has been manufactured. Its characters may be seen in all 



Tyne. Faults upon a much more gigantic scale, which have pushed the broken edges of 

 the ruptured slate rocks more than a mile out of their relative position, pass down some of 

 the old valleys which diverge from the central parts of the Cumberland mountains. 

 These facts afford no support to the indefensible theories of De Luc. Breaks and disloca- 

 tions naturally prepared the way for future valleys; for denuding torrents would more 

 readily scoop valleys out of broken than out of unbroken strata. 



• In the colouring of ttie geological map which accompanies the "Survey of the 

 Yorkshire Coast," the authors have violated their own system; and in the interior of 

 the district have represented a considerable part of the coal formation and alum-shale by 

 one uniform tint. This is unfortunate, and detracts greatly from the value of the map. 

 A geological map ought, for obvious reasons, to represent by appropriate colours those 

 formations only which occupy the surface, without any regard to those formations which 

 are supposed to be below. Had this system been followed, the colour of the coal forma, 

 tion would have been spread over a much larger surface, and the colour of the alum-shale 

 would have been confined to the coast, the western boundary of the district, and the 

 valleys of denudation. 'J'he representation of the alum-shale in Smith's geological map 

 of Yorkshire is a good approximation ; but there are some errors in it, especially in the 

 . representation of the otttttcrs neat Guisboiough. 



