IX. On the Magnesian Limestone and Red Marl or Sandstone 

 of the neighbourhood of Bristol, 



By W. II. Gilby, M.D. 



[Read November 15th, 1816.] 



X H E present communication originates from my having dis- 

 covered 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 al- 

 ways in inclined, and another in horizontal strata. The lowest 

 formation of the first class is the first flcetz 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 flcetz 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 



