1849.] NICOL ON THE SOUTH-EAST OF SCOTLAND. 63 



tributaries of the Tweed follow a course apparently no less devious. 

 The Clyde alone pursues an opposite direction, but many physical 

 phsenomena show that its upper waters formerly joined the Tweed by 

 the low valley near Biggar, and hence even this exception is of mo- 

 dern date, and consequent on some of the most recent revolutions in 

 this district. Thus almost the entire drainage of this mountain-chain 

 flows to one side, so that all its larger river-basins open out to the 

 south. The greater number too of the most elevated mountain-sum- 

 mits range along the northern margin, whilst on the south there are 

 many low hills and undulating ridges. Hence the southern valleys 

 are wider and blend more gradually with the plains than those on 

 the north, where the streams often flow through narrow ravines, or 

 deep notches, cut, as it were, in the steep wall of rock forming the 

 ancient sea cliff. The Gladhouse south of Edinburgh, the Herriot 

 near Dunglass, and the singular ravine crossed by the Peas Bridge at 

 a height of 123 feet, but in other places 150 feet deep and only 50 

 broad, are good illustrations of this peculiarity. 



The line of coast south of Dunglass, where Hutton, Playfair and 

 Hall found many of the most convincing illustrations of those great 

 principles of physical geology which they laboured to establish, still 

 exhibits many remarkable traces of this old Silurian beach-line. 

 From the rocky promontory of Fast Castle, the coast trends westward 

 in a series of bold cliffs, fenced by large fragments and outlying points 

 of greywacke. Near Fala-bank it bends more to the north, and the 

 red sandstone appears on the shore, dipping north-east at 25°, and 

 folded in flat curves round the projecting masses of nearly vertical 

 Silurian strata that project at intervals. The sandstone apparently 

 retains its original position, the dip being due to deposition on a 

 sloping bottom, and not to elevation ; and hence, in following the beds 

 along the coast, they become more and more nearly horizontal. It is 

 highly interesting to observe the sea gradually washing away the 

 sandstone and exposing the ancient beach on which its waves beat so 

 many ages before. 



Notes on the Fossils. 

 Graptolites griestoniensis. Fig. 2. 

 I have given this name to the 



where it occurs. The characters ^^ai^ai^^^^ 



mentioned above, " the oblong 



the stalk," readily distinguish it from any other I have seen described. 

 Each of these serratures has a raised margin dividing it from the axis 

 and from the one that succeeds it upwards. The serratures and axis 

 are about equal in breadth, and together measure Jq inch or under. 

 The length of some fragments is 6 to 8 inches. In fig. 2, a is the 

 natural size ; 5, magnified. 



