THE CLYDE TERRITORY. 409 



hagow gas-coal is one of the important seams, and in the lower portion of 

 this group there are valuable bands of ironstone. The upper group com- 

 prises three beds of limestone, the lowest being the Index so named 

 because it overlies the valuable coal-bearing subdivision of the Carboni- 

 ferous Limestone, the middle being the Calrny, and the upper the Gair or 

 Castlecary band. The same subdivisions are represented in the Ayrshire 

 field, with this important modification, that the lowest or Hurlet limestone 

 has an exceptional development near Beith, where it is about 100 feet 

 thick, and is richly charged with marine organisms. In the northern parts 

 of the same basin, at Dairy, evidence has been obtained of the existence 

 of buried volcanoes, filled with tuff, which Sir A. Geikie assigns to the 

 period of puy-eruptions that followed the volcanic phase of the Calciferoua 

 Sandstone period. He states that in one pit upwards of 115 fathoms of tuff 

 were passed through before the position of the blackband ironstone in 

 that portion of the field was reached by driving levels through the tuff 

 into the adjoining strata. The tuff occurs in patches between which the 

 blackband ironstone is workable. It is evident that volcanic cones must 

 there have been locally developed during the deposition of the ironstone, 

 and that these were eventually buried by higher deposits of the Carboni- 

 ferous Limestone series. 



The Carboniferous Limestone series with the group of workable coals, 

 occurs in various small isolated basins, as, for instance, in the Douglas field 

 in South Lanarkshire, at Dailly in Ayrshire, and to the west of Campbel- 

 town in Kin tyre. Representatives of this series, with its three subdivisions, 

 are met with on the east coast of Arran between Brodick and the Cock of 

 Arran, but the coal-seams there appear in an attenuated form and are not 

 now worked. 



The series of deposits just described points to a succession of marine con- 

 ditions, to the existence of shallow lagoons and mud-flats, and of laiicl- 

 surfaces. It is evident that during the Carboniferous Limestone period the 

 land was steadily subsiding with long intervals of repose, and this irregular 

 subsidence of the land produced overlaps of the strata. For example, the 

 Upper Old Red Sandstone of the Pentland Hills, when traced west- 

 wards, is overlapped by the Calciferous Sandstone, and still further 

 west, near Lesmahagow, the Cement-stone group is in turn overlapped 

 by the Carboniferous Limestone series, which there rests unconformably 

 on the Lower Old Red Sandstone along the south margin of the 

 Lanarkshire field. Similar evidence is obtained in the south part of the 

 Douglas basin, where the Carboniferous Limestone transgresses the Cement- 

 stone group, and reposes on the Lower Old Red Sandstone. Further proof 

 of the southward extension of the Carboniferous rocks and of overlap is 

 found south of Tinto Hill in Lanarkshire, where, in a small outlier, the 

 Calciferous Sandstone disappears, and the Hurlet limestone rests directly on 

 the Lower Old Red Sandstone. Again, between Dalmellington and New Cum- 

 nock conglomerates are associated with the Carboniferous Limestone in such 

 a way as to point to an overlap of that series on to the Old Red Sandstone. 



Throughout the Lanarkshire field the Carboniferous Limestone is over- 

 laid by the Millstone Grit series, which consists of massive, white, yellow or 

 red sandstones associated with thin coal-seams, thin limestones, ironstones, 

 and valuable fireclays. In the south part of the basin, where it is of con- 

 siderable thickness, it occurs to the northeast of Carluke and north of 

 Auchenheath, while in the north part of the basin the fireclays are worked at 

 Glenboig, Gartcosh, and Garnkirk. In the Ayrshire basin the Millstone 

 Grit is absent, at least in some parts of the field, for the Coal-measures rest 

 directly on the Carboniferous Limestone series. On this horizon, however, 

 an interesting band of volcanic rocks has been traced round the east and 

 north sides of the Coal-measures, which, according to Sir A. Geikie, seems 



