Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/221

 east they have a western dip, and on the west an eastern one. In this way the strata tend every where towards a common centre. In the hollow of the basin so formed is deposited a very extensive coal formation, for the particulars of which I refer to the above description. It is sufficient to say that the coal beds and coal measures are always inclined, and when contiguous to the mountain limestone they always dip conformably with it.

It is upon the tops or edges of the inclined strata of these formations that we find the strata of the second division, that is, the horizontal rocks, superimposed in an unconformable position. These horizontal rocks are the same with those occupying so large a portion of the south and south-west of England. The lowest of them are the beds of the red ground, as this formation has been absurdly denominated, consisting of a coarse limestone conglomerate, above which lies a calcareous sandstone, red and white in different places, and then a deposit of red clay, containing gypsum and sulphate of strontian. Above this red ground formation occurs the well known lyas limestone, then the oolites or Bath stones, and lastly the chalk.

The basis of the red ground conglomerate I have generally found to be a common limestone; but being lately at a village on the Bristol Channel called Portishead, I was surprised to find the basis of the conglomerate of a yellow colour, and resembling in appearance some varieties of the Yorkshire magnesian limestone. Upon analysis I found that it did contain a considerable quantity of carbonate of magnesia, the proportion of which varies in specimens taken from different strata. In some strata the basis is so much mixed with sand as to give more than 20 per cent. of insoluble matter, consequently the quantity of carbonate of magnesia is much diminished. The fragmented portions are generally limestone or red sandstone, but we find some strata destitute of sand and fragments,