Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/266

172 PROCEEDINGS OF 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 Lancashire; but until I came to compare the Permian beds of these districts 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 opposite 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 districts, 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