JRevicics — The Survey Memoir on the Scottish Uplands. 511 



described under the headings of rocks, fossils, and conditions of 

 deposition. 



For convenience of description the tableland is divided into five 

 geographical belts, viz. : (i) the Southern Belt, comprising the 

 Wenlock-Ludlow area, which runs along the south-west flanks of 

 the Uplands ; (ii) the Central Belt or the Upland region proper, 

 wliich, forming a continuous band some 25 miles in width, sweeps 

 through the heart of the country from St. Abbs Head to the Mull 

 of Galloway, and is floored mainly by Llandovery and Tarannon 

 rocks ; (iii) the Northern Belt, which ranges from Dunbar through 

 the Leadhills region to the sea-coast north of Portpatrick, and is 

 floored wholly by Ordovician (Lower Silurian) strata ; (iv) the 

 Girvan (and Ballantrae) area; and (v) the area of the inliers of 

 Lanark and the Pentlands. 



It is difficult to say which of the peculiar phenomena of the 

 Upland succession, as now accepted after the researches and 

 discoveries of Messrs. Peach, Home, and M'Connochie, is the most 

 interesting, — the extraordinary tenuity of some of its rock-formations 

 in some districts when contrasted with their extraordinary thickness 

 in others; the striking monotony of their lithology and fauna in 

 one area, as contrasted with the equally striking variety in another ; 

 or, perhaps most remarkable of all, the certainty with which the 

 sequence can be unravelled, in spite of the excessive wrinkling and 

 puckerings which the rocks have undergone. 



Three of the massive Lower Ordovician formations of Southern 

 Britain — the Upper Arenig, Lower Llandeilo, and Upper Llandeilo, 

 inclusive — are now known to be collectively represented in the 

 Uplands by a thin sheet of Eadiolarian chert, only some 70 feet 

 in thickness, which shows through at the surface in numberless 

 anticlinal forms over an area of some 2,000 square miles. Below 

 this chert sheet lies a basement formation — the Ballantrae volcanic 

 series — with a few Middle Arenig graptolites in its higher members ; 

 above it follow shales with Upper Llandeilo graptolites. But the 

 cliert sheet itself is as yet barren of all fossils except Eadiolaria ; 

 so that the rich graptoiite fauna of the Upper Arenig -Lower 

 Llandeilo formations (Llanvirn), so remarkable in Southern Britain 

 and Scandinavia for its characteristic tuning-fork graptolites 

 [Dirlymogrcqjtvs Murchisoni), is in the Uplands conspicuous by. its 

 absence. 



Almost the same extraordinai-y condensation marks the succeeding 

 Upland rock groups — the Glenkiln, Hartfell, and Birkhill black 

 shales (Moffat Series) — which in the Moffat country answer to the 

 Upper Llandeilo, Caradoo, and Llandovery formations of Siluria. 

 They contain, however, the richest and most varied graptoiite fauna 

 known, and their species group themselves in well-marked, con- 

 secutive, paleeontological zones. 



But as we pass outwards from the typical Moffat region to the 

 north-east, north, and north-west, grey shales, grits, and greywackes 

 come in among the black shales, thickening and modifying each 

 zone in turn, comiueuciug from above ; so that finally, when we have 



