THE CLYDE DRAIXAGE AREA. 467 



MILLSTONE GRIT SERIES. 



The officers of the Geological Survey have introduced this series into their 

 maps and memoirs, and have for this purpose drawn the base of the Upper 

 Coal and Ironstone series at the slatyband ironstone, and they seem to 

 have come to the conclusion that there is no Millstone Grit in the Kil- 

 winning area. But the slatyband ironstone does occur there with several 

 argument from the Survey point of view that there is no occurrence of the 

 of the principal coals of the Upper series under it. If this represents the 

 Millstone Grit it will include all the Upper Limestones and the sandstones 

 immediately above and below the Lower and Upper beds. 



UPPER COAL AND IRONSTONE SERIES. 



The strata of this series occupy a very large area in Ayrshire and Lanark- 

 shire, and are associated with the usual clays, shales, and white and grey 

 sandstones of the division. Economically, where at its best, it contains 

 about a dozen workable coals and several ironstones. The fossils generally 

 belong to the genera Anthracomya and Carbonicola, which indicates fresh- 

 water or estuarine conditions. C. robusta often extends in great "scalps" 

 over large areas. In only one locality near Coatbridge do truly marine 

 forms occur in the series, and Lingula is got in one or two places in Ayr- 

 shire. Spirorbis is found attached to plants and shells, S. helicteris occurring 

 crowded into thin bands much in the same way as it is in the cement- 



t/ 



stone (Calciferous) group. Plant-remains occur scattered throughout all the 

 series, or sparingly collected into thin bands known as "fern beds." Several 

 oil-shales occur, and generally yield disjointed remains of fishes and sometimes 

 reptiles. Gyracanthus is got, generally in the roof-shales of some of the 

 coal-beds, ostracoda occur in some of the shales and ironstones, and Belinurus 

 Prestivichia, tfuphoberia, Xylobius, etc., have been obtained sparingly near 

 Kihnaurs. At its upper part this series generally contains thick beds of 

 reddish or purplish sandstones, shales of the same colours, and thin yellow 

 sandstones. In the shale a thin limestone band with Spirorbis rninutus 

 occurs, and also burrows filled with shale, as is well seen at Ballochmyle Scar 

 near Catrine. Other bands of limestone are apparently unfossiliferous. 

 [This Spirorbis limestone also is found at the top of the Lower Coal series 

 of England, near Manchester, there being twelve bands of it, and what 

 have long been known as " The Upper Coal-measures " of Scotland are now 

 considered to be the equivalents of the English Lower Coal-measures.] 



The coal-seams of the Clyde drainage area number in their entirety about 

 seventy, and they are all interesting from a geological point of view. The 

 thin brownish layer the old land- or swamp-surface under them often 

 contains a great variety of beautiful microscopic objects Triletes and micro- 

 spores with remains of scorpions and eurypterids. These fossils are also 

 got in the coals themselves, the hard layers (when not caused by clay) being 

 often masses of their remains. These layers are called " splint." 



The Permian rocks rest unconformably on the Upper Coal and Ironstone 

 series, though sometimes, as in the Snar Valley, on the Silurians. 



