404 THE GEOLOGY OF 



Stinchar, in the Girvan and Ballantrae region, the foregoing divisions of the 

 Silurian system rest unconformably on an eroded platform of Arenig volcanic 

 rocks. South of the Stinchar valley this unconformability disappears, and 

 the sequence resembles that found in the western part of the northern belt. 

 In view of the fossil evidence it has been inferred that this break in the 

 geological record north of the Stinchar valley took place in Upper Llandeilo 

 time. The order of succession and subdivisions of the fossiliferous rocks of 

 Girvan, ranging from the close of the Llandeilo period to Wenlock time, are 

 given in the sequel (p. 429). 



Along the southern margin of the tableland and beyond the limits of the 

 Clyde territory the Tarannon rocks of the central belt pass conformably 

 upwards into Wenlock and Ludlow strata. 



North of the Silurian tableland and within the area occupied by the Old 

 Red Sandstone, various inliers of Upper Silurian rocks are exposed, which 

 have long been familiar to geologists from the discovery of eurypterids 

 in the strata in Logan Water by the late Dr. Slimon of Lesmahagow. 

 Recently fresh interest has been imparted to the study of these inliers by 

 the remarkable fish-fauna obtained from the Ludlow and Downtonian 

 strata. The Upper Ludlow rocks of the Lesmahagow inlier have yielded 

 phyllocarid crustaceans and eurypterids in excellent preservation, together 

 with the remains of a scorpion, a myriapod, two species of fishes (Thelodus}^ 

 and other organisms. 



Next in order there is a group of red and yellow sandstones, conglomerates, 

 shales, and mudstones, forming passage-beds between the underlying 

 Ludlow strata and the overlying Old Red Sandstone, resembling the 

 Downton rocks of Shropshire. The shales and mudstones intercalated in 

 this series have yielded an assemblage of organic remains, in some respects 

 identical with that of the underlying Ludlow rocks, including ostracods, 

 phyllocarid crustaceans, eurypterids, and fishes. The researches of Dr. 

 Traquair have shown the great biological and palaeontological value of this 

 fish-fauna. The discovery of these organic remains in the passage-beds has 

 led to a change in the classification of these rocks by the Geological Survey. 

 Though formerly grouped with the Old Red Sandstone, they are now 

 regarded as forming the highest subdivision of the Silurian system in the 

 south of Scotland. The band of greywacke-conglomerate which is traceable 

 from the Pentland Hills into Ayrshire, save in certain parts of Lanarkshire, 

 is now regarded as the base line of the Lower Old Red Sandstone. For 

 it is not till we reach the horizon of this greywacke-conglomerate that 

 evidence is found of the predominance of materials which have been derived 

 from the Silurian tableland. 



In the northwest part of the tableland the Silurian strata are pierced by 

 various plutonic rocks, of which the largest is the granite mass stretching 

 from Loch Doon to Loch Dee. According to Mr. Teall this mass includes 

 various petrological types, as, for example, granitite, hornblende-granitite, 

 quartz-augite-biotite-diorite, and quartz-biotite-hyperite. 



Towards the close of Downtonian time the Silurian strata of the south of 

 Scotland were elevated and subjected to prolonged denudation, for we find, 

 both in Ayrshire and in the Pentland Hills, the basal conglomerates of the 

 Lower Old Red Sandstone resting unconformably on the folded and eroded 

 edges of the Silurian rocks. In Lanarkshire, however, there seems to be a 

 passage from the one formation to the other. At that period also, the 

 crystalline schists of the Highlands must have formed a prominent land 

 barrier towards the north before the deposition of the Lower Old Red 

 Sandstone sediments. 



The various divisions of the Old Red Sandstone as developed in the Clyde 

 territory are represented in the following table : 



