252 Proceedings of the Royal Physical Society. 
surface, is succeeded by a limestone, denoting a depression which has changed 
the land surface into the bed of a clear sea. The limestone is followed by 
calcareous shales which give place to mixed muddy and sandy deposits and 
at length to sandstone. To show that the shallow sea has been filled up to 
the brim, the sandstone is usually overlaid by fireclay or ganister full of roots, 
and, in turn, by a coal seam representing the rooted vegetation of a land 
surface. The cycle, however, may be arrested at any point owing to minor 
subsidences with shorter intervals of time. A constant recurrence of such 
cycles shows that the land surfaces must have had a very gentle slope 
seawards so that a small vertical downward movement, such as that indicated 
by the short distances between two limestones overlying coals, was sufficient to 
carry what was a land surface far enough from shore to be beyond the reach 
of terrigenous sediment. The complete cycle has taken place more than once 
among the Lower Limestone group, in which is included the Abden Limestones 
which are generally classed as the upper members of the underlying Oil-Shale 
group. 
The Middle group, which is the great source of mineral wealth of 
Scotland, and more extensive than that of the true Coal Measures, shows 
incomplete cycles where the depressions are never sufficient to carry the land 
surface sufficiently far out to sea to produce limestones, although shales with 
marine organisms and estuarine beds occur at one or two horizons. There 
are in some of the coal-fields of the middle division in the present area nearly 
a hundred coal seams, but perhaps only a score of them have been exploited, 
the others being too thin to work hitherto. 
In the Upper Limestone group, about 600-700 feet thick in the Lothians, 
the cycle is more often completed: limestones with true marine fossils recur 
at least four times, and there are several workable coal seams. 
From what has been said above it will be seen that the East of Scotland 
is most favourably situated for studying the local migration of the organisms 
due to the constant change of conditions. Each kind of sediment has its 
characteristic group of fossils whether of marine, estuarine, freshwater, or land 
surface type. 
On studying the mode of occurrence of the organisms to be found in these 
beds, it soon becomes apparent that each different phase of sediment entombs 
its peculiar fauna, which continually recurs with it at all levels. From the 
study of the fish fauna the late Dr Traquair pointed out that black shales, 
vas coal, black-band ironstone, and certain entomostracan limestones, 
sediments that pass insensibly into one another and yield Zingulw, ostracods, 
and estuarine lamellibranchs, contained the same distinct fish fauna of 
estuarine character at all levels in the series. This fauna differs radically 
