LAKES IN RELATION TO GEOLOGICAL FEATUKES 451 
massive white and yellow sandstones, with occasional coal seams. 
Land plants, ostracods, and fish-remains are abundant, indicating 
estuarine conditions ; while the presence of shales with marine fossils, 
and of limestones with corals, crinoids, brachiopods, and gasteropods, 
heralds the marine conditions so prevalent in the lower part of the 
Carboniferous Limestone series. The members of this group reach a 
thickness of several thousand feet in the Lothians and in Fife, but in 
the western districts a part of this sequence of sediments is repre- 
sented by contemporaneous volcanic rocks, to which reference has 
been made. 
The Carboniferous Limestone series is divisible into three groups : 
a lower, comprising several beds of limestone, with sandstones, shales, 
some coals and ironstones ; a middle, containing several workable 
seams of coal, with clay-band and black-band ironstones associated 
with sandstones and shales, but not with limestones ; an upper group 
of three or more limestones, with thick beds of sandstones and coals. 
This triple classification is remarkably persistent throughout the 
Central Lowlands from Fife to Ayrshire, but in the Border region 
beyond the Southern Uplands the middle coal-bearing group is 
poorly developed. 
The characteristic feature of the lower and upper subdivisions of 
the Carboniferous Limestone series is the presence of limestones 
charged with marine organisms. The Hurlet Limestone with its 
underlying coal and alum shale is usually regarded as marking the 
base of the series ; but the boundary is merely an arbitrary one, for, 
as already indicated, marine limestones occur in the upper part of 
the Calciferous Sandstone series. The base of the Upper Limestone 
group is marked by the Index Limestone — so named because it over- 
lies the valuable coals and ironstones of the middle subdivision, and 
its top is represented by the Castlecary seam or its equivalent. 
No contemporaneous volcanic rocks are associated with the 
Carboniferous Limestone series in the western part of the midland 
valley, but in Linlithgowshire they are prominently developed. In 
that district the volcanic eruptions began towards the close of the 
Calciferous Sandstone period, and continued till near the close 
of the Carboniferous Limestone. Occasionally there were quiescent 
intervals, when corals, crinoids, and molluscs migrated to those 
volcanic banks and built up bands of limestones and calcareous shales. 
Next in order comes a succession of white, yellow, or red sand- 
stones merging into grits and fine conglomerates, fireclays of 
economic importance, thin limestones, bands of ironstone, and a few- 
thin coal seams. Where no faults intervene, this formation can be 
traced as a belt of variable width round the margin of the true Coal 
Measures, a feature which is conspicuously developed in the Mid- 
