Carboniferous Period in East Scotland. 257 
fills in the interspaces, showing that the breaking-up and healing must have 
taken place while deposition was still proceeding. May not this expansion, 
due to setting, be sufficient to account for the initiation of “ Reef Knolls” 
so common among the larger masses of Carboniferous Limestone, such as are 
met with in England and the Isle of Man. By this method the reef-building 
organisms would obtain hard surfaces to which they could attach themselves 
and thus increase the “ Knoll.” A “Reef Knoll” on a small scale is seen in 
one of the quarries at Roscobie in Fife. 
MILLSTONE GRIT. 
The Millstone Grit of this region is built up of a succession of sandstones, 
shales, fireclays, thin cementstone or cementstone bands and a few thin coals. 
Caleareous sandstones are its chief feature and give rise to barren country, 
hence the Scottish name of the “Moor Rock,” popularly applied to it in the 
West of Scotland, and the “ Roslyn Sandstone” in the Midlothian Coal-field. 
The Scottish Millstone Grit may be divided into two nearly equal groups, 
the Lower containing the rocks which show recurrences of cementstone or 
limestone with marine fossils, and an Upper Group from which they are 
absent. 
In the area under consideration the beds of the whole series are 
conformable to one another throughout, and show nothing to account for 
the profound Paleontological break which occurs at or near the division 
line of the two groups. Dr Kidston has shown that only one or two species 
of plants pass across this line either up or down. Dr Traquair has for a 
longer time drawn attention to a similar state of matters with regard to the 
species of fishes, especially of those of estuarine habit. The forms beneath 
seem to come up through all the lower subdivisions of the Carboniferous 
rocks, but do not cross the line, while those above it pass on up 
through the Coal Measures. From these facts the line has come to be con- 
sidered, not only a dividing one for the Millstone Grit, but also that between 
the Lower and Upper Carboniferous rocks. As the rocks afford no sign of 
physical break in the region under consideration, some great physical 
change must have taken place outside it, to account for this almost complete 
replacement of the flora and fish fauna by others. Change of climate 
alone seems powerless to bring about so sudden a revulsion. 
As to the conditions of deposit in the lower half of the Millstone Grit, 
we find that the cementstone bands hold a marine fauna similar to that 
which is found in the upper limestones of the underlying Carboniferous 
Limestone Series, while the sandstones and fireclays are of much the same 
nature as those found between the limestones and coals of that series, with 
