202 Proceedings of the Royal Society of Edinburgh. [Sess. 
below the Hurlet Limestone, in the Skolie Burn, Charlestown, Kinghorn, 
and Bilston Burn Sections, the thick Calciferous Sandstone Series has been 
divided into an upper or Oil Shale Group over 3000 feet in thickness, 
and a lower or Cement Stone Group also of considerable thickness. Over 
this area the Blackbyre Limestone or Coral Beef Limestone appears to be 
absent, at least in its typical form, our explanation of its absence being 
that the estuarine conditions which obtained over this part of the area 
were inimical to the existence of such true marine conditions as existed 
when the Blackbyre Limestone was formed. When we reach the St 
Monans Section in East Fife, we find that the Calciferous Sandstone Series 
presents quite a different development from that seen either in the West 
of Scotland or in the Lothians or Western Fife. In place of the widely 
separated marine platforms with comparatively few fossils seen in these 
districts we find a great succession of marine bands rich in organic remains, 
and having a total thickness amounting to about 4500 feet. 
Though the Blackbyre Limestone appears to be absent in the Edinburgh 
district, we are fortunately not left in any dubiety regarding the approxi- 
mate position of this important datum line, for the Hurlet Alum Shale 
with its characteristic faunal association, which has been shown to over- 
lie immediately the Coral Reef or Blackbyre Limestone in the West of 
Scotland, forms a well-marked horizon in the Edinburgh district, enabling 
us to determine with sufficient exactitude the position of this important 
datum line. 
Another important stratigraphical feature which we have to note is 
the local unconformity or break which is generally found to exist at the 
top of the Blackbyre or Coral Beef Limestone. It can be seen over a large 
part of North Ayrshire in the irregular hummocky top of the limestone 
itself, which also sometimes shows a brecciated structure, though on a 
much smaller scale than that seen at St Monans. That the Blackbyre 
Limestone had been upheaved into a land surface over a large part of 
North Ayrshire and the West of Scotland generally is also shown by the 
fact that it often has above it traces of coal or a fireclay containing large 
branching stigmarise, which in many instances appear to have grown on 
the broken-up top of the coral reef. 
Another important stratigraphical feature is the intercalation on various 
horizons in the Lower Limestone Series of thick beds of contemporaneous 
lavas. If the correlation advanced in this paper be accepted, it would 
throw a considerable amount of light upon these volcanic horizons in 
widely separated localities. 
An examination of the comparative vertical sections given on the Plate 
