452 
THE FKESH-WATER LOCHS OF SCOTLAND 
lothian coal-field. In Ayrshire the members of this division are 
absent or poorly developed, and their place is taken bv an interesting 
series of basaltic lavas and tuff's. Evidence of contemporaneous 
volcanic action on this horizon has also been obtained in Arran and 
in Campbeltown. 
From a study of the fossil flora Dr Kidston has arranged the 
Carboniferous rocks in two divisions, the boundary between the two 
being drawn in the lower part of the Millstone Grit in the Central 
Lowlands : a classification which has been reached independently by 
Dr Traquair from the evidence supplied by the fossil fishes. Reference 
ought to be made to the remarkable assemblage of lamellibranchs, 
resembling the lamellibranch fauna of the Coal Measures of Nebraska 
and Illinois of North America, which has been obtained from the lower 
part of the Millstone Grit and described by Dr Wheelton Hind. 
The Coal Measures, which form the highest division of the 
Carboniferous system, comprise an upper group, consisting of red 
sandstones, shales, fireclays, marls, and a band of Spii^orhis limestone, 
and a lower group of great economic value, containing numerous 
coal seams, clay-band and black-band ironstones, bituminous shales, 
fireclays, and white and grey sandstones. From an examination of 
the fossils it is evident that, during the deposition of the true Coal 
Measures, fresh or brackish water conditions must have prevailed 
generally throughout the Scottish basins. Marine bands with 
brachiopods do occur, but they are rare. The constant repetition of 
coal seams with sandstones, shales, and ironstones shows that land 
conditions must have been in the ascendant, followed at intervals by 
slight submergence. 
Within the Silurian tableland of the Southern Uplands a basin 
of Carboniferous strata rests unconformably on the Silurian rocks 
and forms the Sanquhar coal-field. From the relations of the younger 
strata to the older series it is clear that long before the Coal Measures 
were laid down the old tableland must have been carved into hills 
and valleys ; in short, Nithsdale must have been a valley in Carbon- 
iferous time. At the south end of the Sanquhar basin there are 
some isolated patches of strata belonging to the Carboniferous Lime- 
stone series, which, in the adjoining basin of Thornhill, are much 
more largely developed. When traced northwards these disappear, 
till in the Sanquhar basin the Coal Measures rest directly on the 
old floor. Such evidence points to the irregular subsidence of the 
old tableland of the Southern Uplands. 
Evidence has been obtained of the former extension of Carbon- 
iferous rocks within the Highland area. At Ardtornish, Morvern, 
Professor Judd discovered sandstones and shales with thin seams of 
coal which yielded Carboniferous plants, but we have no means of 
