410 THE GEOLOGY OF 



to mark the latest Carboniferous volcanoes. Where most fully developed, 

 these rocks consist of bedded basalts and tuffs, reaching a thickness of 60(> 

 feet. In the Geological Survey map (sheet 14) they form a conspicuous 

 feature, intervening between the Carboniferous Limestone and the Coal- 

 measures along the east side of the field near Kilmaurs, but they thin away and 

 appear as a narrow band between Kilwinning and the shore section at Salt- 

 coats. It is interesting to note that a band of fine tuff, occupying the same 

 horizon at the base of the Coal-measures, has recently been detected by Mr. 

 Gunn in Arran, and a similar band has been found in the strip of Carboni- 

 ferous rocks on the west side of Loch Ryan. No contemporaneous volcanic 

 rocks occur on this horizon in the Lanarkshire basin. 



Though the massive sandstones of the Millstone Grit imply the accumula- 

 tion of coarse sediment near the land, still the shales, ironstones, and impure 

 limestones in this division have yielded plants, brachiopocls, crustaceans, 

 fishes and other organisms, pointing to the existence of marine conditions. 



The Coal-measures overlying the Millstone Grit are divisible into two 

 groups, a lower, containing numerous valuable coal-seams, ironstones, fire- 

 clays, bituminous shales, sandy shales, and sandstones, and an upper group 

 of red sandstones, fireclays, marls, and thin coal-seams, resting, probably 

 with a slight unconformability, on the lower. The vertical section of the 

 Clyde coal-field, published by the Geological Survey, shows eleven coal- 

 seams and seven seams of ironstone in the lower group, in the central portion 

 of the basin. (" Explanation " of sheet 23, one-inch Geological Survey Map, 

 page 36.) Some of the seams rest on a bed of fireclay, representing the old 

 land-surface, while others have no underclay ; the latter may be due to the 

 drifting of vegetable matter. The red sandstone, group overlying the 

 valuable mineral field in Lanarkshire extends up the valley of the Clyde from 

 Cambuslang by Both well to Hamilton, being admirably seen in the quarries 

 near Uddingston. These two groups are represented in the Ayrshire field. 

 Quite recently Mr. Gunn has detected representatives of the Coal-measures 

 in Arran, in tributary branches of the Sliddery Water, which have yielded 

 two species of Carbonicola, and several species of fishes appearing to be 

 characteristic of the Upper Carboniferous formations in Scotland, together 

 with specimens of Lepidodendron Velt/ieimictJium, which, according to Mr. 

 Kidston, has never yet been found above the Carboniferous Limestone. The 

 shales, ironstones, and other sediments yielding these fossils are there 

 associated with fine tuffs, coarse volcanic ash, and lavas, to which reference 

 has already been made in connection with the volcanic zone at the base of 

 the Coal-measures in Ayrshire. (Summary of Progress, Geological Survey, 

 1897, page 113.) From the constant succession of coal-seams, ironstones, 

 sandstones, and shales throughout the Coal-measures it may be inferred that 

 land conditions alternated with slight submergence during the accumulation 

 of these deposits. That the irregular sinking of the Silurian tableland led to 

 overlaps during the Coal-measure period is conclusively proved in the 

 Sanquhar District, and near Loch Ryan. For in the former region both 

 the Calciferous Sandstone and the Carboniferous Limestone are overlapped, 

 and the Coal-measures rest unconforrnably on Silurian strata in the Sanquhar 

 basin. Again, on the west shore of Loch Ryan the shales occurring in the 

 strip of Carboniferous rocks, which there rest on the Silurian floor, have 

 yielded plants characteristic of the Coal-measures. 



Reference must be made to the researches of Mr. Kidston, who, by means 

 of the plants, has pointed out the existence of a great palaeontological break 

 at the base of the Millstone Grit. A similar conclusion has been inde- 

 pendently arrived at by Dr. Traquair from an examination of the fishes. 



Numerous dykes and sills pierce the Carboniferous strata of the Clyde 

 territory. The dykes, which may be referred to the period of the plateau - 

 eruptions of the Calciferous Sandstone period, consist mainly of trachyte or 



