40 Rev. A. Sedgwick on the Geological Relations and 



account for the extraordinary difference between the conglomerates overlying 

 many portions of the coal districts in the south-western parts of England, and 

 that well developed and extensive formation of magnesian limestone which it 

 will be my object in this paper to describe. 



If the preceding views be correct, it must obviously be impossible to form 

 any just estimate of the important deposits which connect the great coal-for- 

 mation with the beds of the superior order, without many independent details 

 derived from situations which are remote from each other, and in which the 

 relations of the deposits are well exhibited. The history of the new red sand- 

 stone in the western coal districts may (after the admirable details published 

 in a preceding volume of the Society's Transactions,) now be regarded as 

 complete. The following details may, I hope, afford the materials for ap- 

 proximating towards a more perfect history of the same formation in some 

 other parts of England. 



§ 2. External character of the country through lohich the formation ranges, S^c. 



In the south-western coal-fields the magnesian conglomerates do not often 

 by themselves form any important feature in the country. Frequently they 

 are found immediately associated with the next superior formations of lias and 

 oolite, and are the base of the horizontal deposits which rest upon the edges 

 of all the older rocks*. To the formation of magnesian limestone I am about 

 to describe, none of these characters can be applied ; for it is separated from 

 all our oolitic formations by the great plain of the new red sandstone, rests ex- 

 clusively on beds of the carboniferous order, and in almost every part of its 

 course produces a striking influence on the external character of the country. 



The western boundary of the formation generally presents a lofty escarp- 

 ment overlooking the lower portions of the neighbouring coal districts ; and 

 the top of this escarpment preserves nearly the same level through many parts 

 of its range. In consequence of this structure, the profile of the formation, 

 when seen from the west, may often be represented by a number of nearly 

 horizontal lines, separated from each other by intervals indicating the valleys 

 of denudation. This statement, though generally applicable to the confi- 

 guration of the country, admits of some exceptions. In several places the 

 escarpment above mentioned disappears altogether ; and in some parts of the 

 county of Durham there is no continuous terrace ; but the visible boundary of 

 the limestone is represented by an obscure chain of low round-topped hills. 



* See the sections of the south-western coal districts of England. (Geol. Trans. 2nd Series, 

 vol.!. Part 2, Plate XXXII. and XXXV.) 



