Dr. Gilby on the Magncsian Limestone , &c. 211 



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 mea- 

 sures 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 forma- 

 tions that we find the strata of the second division, that is, the hori- 

 zontal rocks, superimposed in an unconformable position. These 

 horizontal rocks are the same with those occupying so large a por- 

 tion 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 surprized to find the basis 

 of the conglomerate of a yellow colour, and resembling in appear- 

 ance some varieties of the Yorkshire magnesian limestone. Upon 

 analysis I found that it did contain a considerable quantity of car- 

 bonate 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 di- 

 minished. The fragmented portions are generally limestone or 

 red sandstone, but we find some strata destitute of sand and frag- 



