IX. On the Magnesian Limestone and Red Marl or Sandstone of the neighbourhood of Bristol.
By W.K. Gilby, M.D.
[Read November 15th, 1816.]
THE present communication originates from my having discovered
in this neighbourhood the magnesian limestone which
exists so extensively in the North of England; and as its position
with regard to the other formations is remarkably distinct, we have
here at least an opportunity of ascertaining its true geological rank,
concerning which much uncertainty has prevailed in other quarters.
In the description which I have given in the Philosophical Magazine of the geology of this neighbourhood, it will be seen that we have here two grand divisions of rocks, one occurring always in inclined, and another in horizontal strata. The lowest formation of the first class is the first flœtz or old red sandstone, exceedingly well characterized. It is only in certain tracks that this rock constitutes hilly ground so as to be visible to the eye: but the first flœtz or mountain limestone which rests upon it, has a very extensive range, describing in its course an irregular ellipsis, the direction of which it will here be unnecessary to repeat. I may however remark that analogous to what has been observed with regard to the ellipsis of mountain limestone in South Wales, the dip of its strata varies remarkably in different parts of its course. At the north the strata dip south, and at the south, north. On the
