Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/223

 flows over beds of white and red calcareous sandstone containing gypsum, which rests upon magnesian limestone. Now this gypseous calcareous sandstone may be said to characterize the red ground, therefore the magnesian limestone in that district may be safely referred to that formation, and consequently may be regarded as the rock next in succession to the Durham coal deposit. This indeed might be collected from the imperfect sketch Dr. Thomson has given us of its occurrence in that quarter. In a late number of the Philosophical Magazine, it is said that the coal formation of Whitehaven is covered at St. Bees Head by bituminous clay, over this is a limestone containing magnesia and iron, and above this is a red sandstone, connected with which is clay, marl, and gypsum. This it will be perceived is almost an exact account of the succession of the red ground strata in this quarter. In Derbyshire we are informed that the magnesian limestone lies in an unconformable and horizontal position over the inclined strata, after the manner of its occurrence in this quarter, it therefore plainly belongs to the same formation, the red ground.

It may seem at first very remarkable that the basis of the limestone conglomerate should be in one place merely a common limestone, and that in another spot, not far distant, it should contain a considerable proportion of magnesia. To those however who have seen how widely the same rock formation, nay even the same stratum, will vary in its colour, hardness, and general structure in different parts of its course, this will cease to be a matter of wonder. In attempting to explain these singularities, it seems to me that we must resort to one of two suppositions: we must either conceive that the fluid menstruum during the deposition of any particular and extensive formation, must have contained in different places different chemical ingredients; or we must conclude that the alteration