172 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Feb. 10, 



engaged on a memoir for the Geological Survey on the Triassic and 

 Permian rocks of the central counties ; and it was when describing 

 and comparing the Permian rocks of Lancashire with those of 

 Shropshire and the midland counties that I was forcibly struck by 

 the dissimilarity in structure and composition between the beds of 

 these two tracts of England. I had, indeed, been aware of these 

 differences previously, having passed several years in the survey of 

 these rocks over parts of the central counties, Shropshire and Lanca- 

 shire ; but until I came to compare the Permian beds of these dis- 

 tricts with each other on the occasion referred to, I had not perceived 

 the full force of the fact that there are two types of beds, and two 

 only, over the whole region from Warwickshire to Lancashire and 

 Cumberland, and that the divisional line must lie somewhere in the 

 region of the central plain of Cheshire. 



Having arrived at this conclusion, it was natural I should seek 

 for the cause of this dissimilarity between the Permian beds on the 

 north and the south of the plain; and there were at first sight 

 evidently three modes of explanation. 



1. In the first place, it might be supposed that the whole of the 

 beds were deposited contemporaneously in one continous basin, and 

 that the differences in mineral structure and composition arose from 

 differences in the nature of the sediment brought down from op- 

 posite regions of land, and poured into the northern and southern 

 portions of the basin respectively. 



2. Or we might consider the beds of the two districts to be 

 relatively of different ages, those of Lancashire and the north 

 being more recent than those of Shropshire and the midland 

 counties. 



3. Or, lastly, we might suppose the beds of the two districts to be 

 contemporaneous, but deposited in different basins, owing to the 

 existence of a divisional ridge or barrier of the older rocks, such as 

 is considered to have existed in the region of the Bristol Channel in 

 the Devonian period. This is the only explanation which appears 

 to me sufficient and tenable ; and I venture to lay the reasons for 

 this view before the Society in as concise a manner as the subject 

 seems to admit of. 



Before entering on this part of the inquiry, it will be necessary 

 to present a brief description of the Permian rocks of the two dis- 

 tricts, showing the distinctive characters of the two types I have 

 referred to. But, in so doing, it is far from my intention to give a 

 detailed description of the Permian districts of Central and Northern 

 England, as ample accounts are already published, and to do so 

 would therefore be but vain repetition. I shall therefore content 

 myself with passing briefly in review the nature of the rocks as far 

 as is necessary for my purpose, referring while so doing to the 

 works of the eminent geologists who have written upon this branch 

 of British geology. 



Permian Beds of the Salopian Type. 

 Distribution. — The general characters presented by the beds of 



