C— GEOLOGY. 61 



Silurian is widespread in this mountain element and shares in the 

 intense corrugation and frequent cleavage of its Ordovician substratum. 

 Lower Old Red Sandstone occurs in Anglesey and the Cheviots and 

 between Girvan and Edinburgh, and is markedly later than the major 

 deformation of the Silurian. Still, both in Anglesey and near Girvan, 

 Lower Old Red Sandstone has suffered pronounced deformation, and in 

 the former locality has actually been cleaved. 



Near Girvan we find, in addition to the post-Silurian unconformity, 

 another of intra-Ordovician date, sufficiently important to bring Upper 

 Llandeilo conglomerates on to Arenig plutonic intrusions. This earlier 

 unconformity disappears with amazing rapidity towards the south-east ; 

 but north-westwards it increases in scope, while in the same direction the 

 post-Silurian unconformity fails. 



The evidence for these propositions lies partly in the Southern Uplands 

 and partly in exposures to the north-west. The interpretation of the 

 Southern Uplands is one of the miracles of Science. We owe it to Lap- 

 worth, an English schoolmaster attracted to Galashiels by the charm of 

 Scott's romances. During the seventies of last century Lapworth demon- 

 strated that the hitherto despised graptolites furnish an extraordinarily 

 sensitive time-scale for Ordovician and Silurian stratigraphy. This led 

 him on to the discovery that many of the rock groups that pass with 

 broken complication through the tightly compressed steep isoclinal folding 

 of the district change profoundly in thickness and character from south- 

 east to north-west. The total thickness of the Upper Llandeilo, Caradoc, 

 and Llandovery at Moffat in the centre of the Southern Uplands is given 

 by Peach and Home as 220 feet, consisting of black graptolitic shale and 

 unfossiliferous mudstone. At Girvan, which is only 25 miles to the 

 north-west in cross-strike measurement, these same formations are 

 reckoned as more than 4,800 feet thick, and their constituents include con- 

 spicuous conglomerates, grits, flags, grey shales, shelly beds and one 60-foot 

 limestone, in addition to subordinate intercalations of black graptolitic 

 shales. Careful examination of many intermediate exposures, afiorded 

 by folds one behind another, has allowed the details of this transformation 

 to be deciphered. The coarse deposits mark an apj^roach to a coast line 

 lying to the north-west, and their material contains much recognisable 

 debris of Arenig cherts, lavas and intrusions that must have formed part 

 of a land surface in that direction. At each successive period, starting 

 with Upper Llandeilo, the coarse sediment pushed farther and farther 

 south-eastwards across the sea bottom. In Tarannon times it had reached 

 beyond Moffat ; and to find exposures of a complete black graptolitic 

 representation of this particular period one has to travel to the English 

 Lake District. 



When it is remembered that this variation of facies is combined with 

 incessant isoclinal packing and accompanying dislocation, and that the 

 grassy Southern Uplands are as devoid of geological features as are the 

 Chalk Downs of Sussex, Lapworth's triumph fully exonerates the failure 

 of his predecessors. 



From the great thickness of shallow-water marine sediments, deposited 

 during Ordovician-Silurian time near the northern edge of the Southern 

 Uplands, we may deduce a corresponding long-continued subsidence of 



