234 STRATIGKAPHICAL GEOLOGY 



the Firth of Clyde, a distance of about 120 miles. For the greater 

 part of its extent the breadth of outcrop is about 20 miles, but it 

 narrows in Dumbarton where the thickness is probably less. 



On the north-west side it is cut off by the great boundary 

 fault ; but in Perthshire and the west of For far this is not a single 

 but a double line of fault, and there are small tracts of the Old 

 Ked rocks beyond the second fault-line, resting on the schists and 

 quartzites of the Highland complex (see Fig. 79). These outliers 

 prove that the formation had originally a considerable extension 

 to the northward over the rocks of the Central Highlands. If, 

 indeed, the thickness of the series in Perthshire was as great as it is 

 on the eastern coast, the height of the mountains which then 

 formed the Highland range must have been well over 12,000 feet, 

 and it is clear that the present hills are but the denuded stumps of 

 a Devonian Alpine Range. 



Further, since the lower slopes of this range seem to have been 

 gradually buried beneath the increasing pile of red sandstones and 

 lava-flows, it is not surprising to find a large tract of volcanic 

 rocks in Lome associated with sediments which prove the whole to 

 be of Lower Old Red age. These rocks form the plateau of 

 Lome, which consists of a succession of terraces and escarpments, 

 rising to 1700 feet above the sea and composed of felsitic and 

 andesitic lavas with occasional layers of agglomerate, conglomerate, 

 and red shale. At the base, and exposed along the coast near 

 Oban and in Kerrera Island, are shales and flagstones with a 

 boulder conglomerate at the base from 20 to 200 feet thick. These 

 beds have yielded remains of Cephalaspis, Pterygotus, Kampecaris, 

 and the plant Psilophyton. Still farther north in Glencoe is 

 another patch of andesites with some shales in which Psilophyton 

 and Pachytheca have been found. 



On the southern side of the great Lowland syncline the Lower 

 Old Red crops out in a more irregular manner, and is only exposed 

 in disconnected areas in Ayrshire, Lanark, and the Pentland Hills. 

 The beds described on p. 177 as forming the highest member of 

 the Silurian System were formerly classed with the Old Red 

 Sandstone, but the base of the latter is now taken at a band of 

 conglomerate containing greywacke pebbles which is recognisable 

 in all three districts. Near Lesmahagow (Lanark) this con- 

 glomerate rests with apparent conformity on the red Silurians, 

 and is succeeded by chocolate -coloured sandstone containing 

 Cephalaspis Lyelli, but in Ayrshire and in the Pentlands the basal 

 conglomerate is markedly unconformable to this Silurian Series. 18 

 Fig. 59 is a diagram section showing the general relations of these 

 beds in the Pentland district. 



