THE SILURIAN ROCKS IN THE SOUTH OF SCOTLAND. 

 BY B. N. PEACH, F.R.S. ; J. HORNE, F.R.S. ; and 



A. MACCONOCHIE. 



INTRODUCTION. 



THE broad tableland of the Southern Uplands, stretching from the great 

 midland vallev southwards to the Solwav Firth and the Cheviot Hills is 



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composed mainly of Silurian strata. Along its northern margin, the 

 Silurian tableland is bounded by a powerful fault which lets down the Old 

 Red Sandstone and Carboniferous rocks of the central lowlands ; while on 

 the southeast side, in the region extending from the Merse of Berwickshire 

 to the Solwav, the Old Red Sandstone and Carboniferous strata rest uncon- 



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formably on the upturned and eroded' Silurian rocks. The scenery of these 

 pastoral uplands is remarkably uniform save where these older Palaeozoic- 

 rocks have been pierced by large granite masses, as for example between 

 Criffel and Bengairn and between Loch Dee and Loch Doon. Over much 

 of the tableland there is little variation in the lithological characters 

 of the strata, as they consist mainly of grits, greywackes, flagstones 

 and shales. There are, however, certain bands of black shales richly 

 charged with graptolites, by means of which Professor Lapworth demon- 

 strated the true order of succession of the strata. Everywhere the rocks 

 have been thrown into innumerable folds frequently inverted and traversed 

 by normal and reversed faults the axes of which run from southwest to 

 northeast, that is, parallel to the long axis of the tableland. 



Along the- southern margin of the tableland there is a strip of Wenlock 

 and Ludlow rocks. In the central part of the chain, there is a belt- 

 upwards of twenty miles in width composed mainly of Llandovery and 

 Tarannon rocks with inliers of the Moffat series of graptolite shales, and 

 extending from the Mull of Galloway by Moffat to St. Abb's Head. In the 

 northern portion, stretching from the northern slopes of the Lammermuir 

 Hills southwest by the Leadhills to Loch Ryan and Portpatrick there is a 

 great development of Arenig, Llandeilo, and Caradoc rocks. The Girvan 

 region which embraces strata ranging from Arenig to Wenlock time is of 

 special interest owing to the abundance of organic remains found on certain 

 horizons. Beyond the northern limits of the Silurian tableland, and within 

 the area occupied by the members of the Old Red Sandstone and Carbon- 

 iferous systems in Lanarkshire and Ayrshire, there are various exposures of 

 Upper Silurian rocks, of Wenlock, Ludlow and Downtonian age. 



Though the portion of the Silurian tableland which falls to be described 

 here is restricted to that embraced in the Clyde basin and adjoining the 

 Firth of Clyde, the following table has been prepared to show the classifi- 

 cation of the Silurian rocks throughout the Southern Uplands : 



