560 Repoi^ts & Proceedings — EdinburgJi Geological Society. 



a parallel-sided trough containing the liquid to be determined, in 

 which is immersed a right-angled prism of known index near that 

 of the liquid. Two images of the signal are formed, and the angular 

 distance between them is read on the eye-piece scale ; this reading 

 is proportional to the difference of index between the liquid and the 

 prism. The scale division has the same value, whatever the index 

 of the prism used. 



Edinburgh Geological Society. 



October 20, 1920.— Mr. E. B. Bailey, M.C., B.A., F.G.S., F.R.S.E., 



President, in the Chair. 



"Notes on the Structure, Character, and Relationship of the 

 Lower Carboniferous Limestones of St. Monans, Fife." By David 

 Tait and James Wright. 



In this communication the authors gave a description, with 

 diagrams, of the lower members of the Carboniferous Limestone 

 Series which are exposed on the shore front in, and to the east, of 

 St. Monans. Additional fossil lists from certain horizons were also 

 given. Some of the points brought forward have not hitherto been 

 published. Evidence was adduced to show that the " white " lime- 

 stone of "^t. Monans undergoes a remarkable change from a coralli- 

 f erous limestone at high-water mark at Coal Farm to a limestone of 

 quite normal character at St. Monans Burn. Attention was also 

 drawn to the difference in the cover of this limestone, and the change 

 it undergoes towards low-water mark at Coal Farm. The " pseudo- 

 brecciated " limestone of St. Monans probably owes much of its 

 brecciated character to faulting. Besides the Coal Farm exposure, 

 this bed also appears in its proper position on the west side of the 

 syncline at St. Monans Burn. In all the positions examined, where 

 it is undisturbed it may be described as a limestone of normal 

 type ; where it has been subjected to faulting, as at Coal Farm and at 

 places on the western limb, it shows the " pseudo-brecciated " 

 structure of the high-water exposure of the Coal Farm section. The 

 overlying shales at such places are much disturbed and slickensided. 



The higher limestones in the sequence retain their characters on 

 both sides of the fold, but the intervening strata on the west are on 

 the whole thicker than those on the east. 



Description was also given of a remarkable limestone below the 

 " white " Hmestone of Coal Farm. At high water this bed is normal 

 and only 6 inches thick. Seawards it passes under a coralliferous 

 layer, which gradually attains a thickness of 2 ft. 9 in., and from 

 this point tapers away altogether. For a distance of 4 yards there 

 is no limestone. It begins again as a thin coral bed of the same 

 nature, thickens out to 18 inches, then dies away near low-water 

 mark. 



Attention was drawn to the nodular and other characters of these 

 limestones and the evidence they afiord of the peculiar conditions 

 under which they were deposited. 



