THE CLYDE TERRITORY. 403 



stated that the evidence points to continuous sedimentation from Arenig to 

 Ludlow and Downtonian time, save in the northwest region where local 

 unconformabilities occur. 



The northwest portion of the Silurian tableland embraced in the Clyde 

 territory furnishes striking proof of the lateral variation of the Silurian 

 strata as the observer passes from the central Moffat region northwestwards 

 to Leadhills and Girvan. The land from which the sediment was derived 

 lay to the north, and hence there is a gradual increase in the thickness of 

 several divisions of the Silurian system as we approach the Girvan area. One 

 prominent rock-group preserves, with rare exceptions, its uniform lithological 

 characters throughout the uplands. It consists of cherts and mudstones 

 partly of Upper Arenig and partly of Lower Llandeilo age, which, where not 

 deformed or altered by intrusive igneous masses, are richly charged with 

 Radiolaria. The mudstones contain hingeless brachiopods and other 

 organisms. The cherts, which have been formed from a true radiolarian 

 ooze, and the mudstones, imply an oceanic phase of sedimentation. Their 

 age is clearly defined, for throughout the uplands, where exposed on 

 anticlinal folds, they are overlain by the Glenkiln shales with graptolites of 

 Upper Llandeilo age ; and they rest on volcanic rocks containing, in the 

 Girvan area, cherty mudstones and graptolitic shales, yielding Middle 

 Arenig graptolites (see general Table of Silurian strata, pp. 424, 425). 



By far the greatest development of the Arenig volcanic rocks occurs near 

 Ballantrae, where they consist of diabase and diabase-porphyrite lavas, 

 agglomerates and tuffs, pierced by various plutonic masses, including 

 serpentine, gabbro, dolerite, and granite. But they reappear on numerous 

 anticlines between Abington and Leadhills in Upper Clydesdale, where 

 they are overlain by the Radiolarian cherts and Glenkiln-Hartfell black 

 shales. 



The subdivisions of the Moffat series, established by Professor Lapworth 

 in the central portion of the tableland : viz. Glenkiln shales (Upper 

 Llandeilo), Hartfell shales (Caradoc), and Birkhill shales (Llandovery), 

 imply conditions of deposition near the verge of sedimentation, for they 

 consist of alternations of black shales, cherty bands and mudstones with rare 

 intercalations of coarser sediment. The total thickness of these divisions of 

 the Silurian system in the Moffat region does not exceed 300 feet, and when 

 traced northwest to the limit of the tableland they are represented in the 

 Girvan region by upwards of 5000 feet of strata (see general Table of 

 Silurian rocks, pp. 424, 425). These lateral variations may thus be briefly 

 summarised. Northwards from the central Moffat area the Birkhill shales 

 are gradually replaced by coarser sediments, till along the northern margin 

 of the Tarannon belt they are represented by grits, greywackes, and shales 

 with thin carbonaceous seams yielding dwarfed representatives of Lower 

 Birkhill graptolites. In like manner in the northern area, the Hartfell 

 black shales undergo a similar change, and their characteristic graptolites 

 appear, either in thin black seams interleaved in flaggy shales, or in dark 

 sandy shales. The barren mudstones of the central Moffat region are 

 represented in the Leadhills region by grey and blue micaceous shales 

 (Lowther shales), 'grey wackes and grits, with lenticular masses of limestone, 

 which at Wrae and Glencotho are associated with volcanic rocks. Similarly 



M 



the Glenkiln shales lose their normal lithological characters in the northern 

 belt, and their characteristic graptolites are found in thin dark seams in 

 sandy shales, interbedded with greywackes and shales. 



In the Girvan region these lateral modifications of the strata are more 

 strongly marked, as shown by Professor Lapworth, for the Moffat series is 

 there represented by a vast thickness of conglomerates, grits, greywackes, 

 flagstones, shales, mudstones and limestones (Barr series, Ardmillan series, 

 Newlands series of Lapworth, see p. 429). To the north of the valley of the 



