] 30 Reports and Proceedings— Royal Society of Edinburgh. 



younger members of the Survey, Mr. E. L. Jack, viz., the occurrence of 

 a bed of fossiliferous conglomerate, which has been traced for about five 

 miles among the Lower Silurian rocks of the Leadhills. The fossils 

 have not yet been examined, but there seems much probability thab 

 this bed will prove to be a prolongation of the well-known Wrae 

 limestone, and thus define the age of the strata of the Leadhill dis- 

 trict. A large area of Old Bed Sandstone has now been examined, 

 including the whole region between the Pentland Hills and the 

 south-west of Ayrshire. Three distinct divisions have been ascer- 

 tained — an upper series, graduating upwards into the Carboniferous 

 formation, and best seen in the eastern parts of the kingdom, a 

 middle group, extensively developed, to the south of the town of Ayr, 

 and a lower group, which reaches an enormous thickness, in the 

 centre of the Lowlands, and passes down into the Upper Silurian. In 

 this lower division, as it approaches its north-eastern limit, a re- 

 markable local unconformity has been ascertained by the recent 

 researches of Mr. B. IS". Peach and Mr. Geikie. In the Pentland 

 Hills, the conglomerates and sandstones rest upon the vertical 

 abraded edges of the Upper Silurian rocks ; while only a few miles 

 distant, in the parish of Lesmahagow, there is no such break, but 

 the one series passes regularly into the other. The Pentland 

 Hills, therefore, contain two groups of strata, both belonging 

 to the Lower Old Eed Sandstone, but the one lying quite uncon- 

 formably on the other. The great development of contempora- 

 neous igneous rocks associated with these strata, may possibly 

 be connected with the origin of this local break in the suc- 

 cession of the deposits. Much attention has been bestowed by 

 the Survey during the last few years upon the lower members of 

 the Carboniferous system. The result has been a twofold division 

 of that hitherto vaguely defined set of strata known as the " calci- 

 ferous sandstones," between the top of the Old Eed Sandstone and 

 the base of the Carboniferous Limestone. New light has conse- 

 quently been thrown upon the history of the earlier portions of the 

 Carboniferous period in Scotland. The lower part of the calciferous 

 sandstones consists of a thick but variable group of red sandstones, 

 extensively developed in Ayrshire, Arran, and Bute, and stretching 

 across the island to the coast of Haddington and Berwick. Between 

 these Lower Sandstones and the base of the Carboniferous Lime- 

 stone, comes a group of strata possessing much interest from the 

 variations which it exhibits in thickness, and in the nature of its 

 component strata. In some districts it is altogether absent, and 

 then the Carboniferous Limestone and the Eed Sandstones come 

 together. But again, at no great distance, it reappears, and soon ac- 

 quires a considerable thickness. In the western part of the country 

 it consists of grey and white Sandstones, pale grey, green, or 

 mottled marls and shales, with bands of cement-stone, sometimes 

 with dark shale and ironstone. The Ballagan beds of the Campsie 

 Hills belong to this group. In the eastern half of the country, 

 throughout the Lothians, it is made up of thick white sandstones, black 

 shales, ironstones, some beds of Limestones, and even of coal, its 



