62 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 16, 



resemblance of the steep slope of the greywacke hills to a sea cliff, 

 somewhat softened by degradation in the long lapse of ages, can 

 scarcely be overlooked by the most careless observer. 



And such, I believe, has been the origin of the peculiar features of 

 this boundary -line. The Silurian strata evidently extend much farther 

 north below the more recent deposits than their boundary on the map. 

 This is proved by the fragments, covered un conformably by the old 

 conglomerates and sandstones, which are exposed at several points in 

 the Pentland hills where the deeper masses have been forced up by 

 the igneous rocks in that chain. Such an extension of the Silurian 

 beds is also required to balance, as it were, the southern side of the 

 anticlinal arch, stretching down to the border of England. Hence I 

 conclude, that whilst the southern half of the greywacke rocks was 

 being cut into valleys by river action, the northern margin was ex- 

 posed to the wasting influence of an open sea, which has planed down 

 that rocky bed on which the newer formations of the central trough 

 of Scotland have been deposited. It has only been near the conclu- 

 sion of the Devonian period that conditions were again established in 

 the southern part of the central valley of Scotland* permitting detritus 

 to accumulate round the ancient shores. This accumulation has then 

 gone on continuously during the whole carboniferous period — red 

 sandstones passing gradually into white, these becoming mixed with 

 shales and then with hmestones, as the waters freed from the iron- 

 peroxide became more favourable to the growth of corals and crinoids. 

 Later the calcareous deposits decrease in abundance, and shales and 

 sandstones alternate with seams of coal. During this period the land 

 must have been alternately above and below water, the upright trees 

 seen in many places having grown during the former; the large trunks, 

 forty feet long or more, exposed in the sandstone quarries near Edin- 

 burgh, having been drifted into the basin during the intervals of sub- 

 mergence. 



The influence of these ancient revolutions on the actual physical 

 geography of the country, particularly the direction of river drainage, 

 deserves notice. On drawing a line along the watershed of the moun- 

 tain chain, separating the rivers that flow south from those that reach 

 the sea on the north of the axis, it is seen that the latter are compa- 

 ratively insignificant. The division-line falls either very near, or even 

 beyond, the northern boundary of the chain and of the Silurian rocks. 

 Many of the streams that rise in the newer formations of the central 

 district intersect the whole mass of older deposits on their way to the 

 sea. Thus the Nith has its source in the coal formation of Ayrshire, 

 within twelve miles of the Firth of Clyde, but turns south and falls 

 into the Solway, after passing through the whole ridge of Silurian 

 mountains, elevated in many points from 2000 to 3000 feet above the 

 sea. Further west, some of the smaller streams, as the Cree and Ken, 

 illustrate the same peculiarity ; and on the east, the Lyne and other 



* On the north side of this trough or valley, in Perth, Forfar, and Kincardine 

 shires, these deposits have begun rauch earlier. This is shown by the great ex- 

 tent of the old red sandstone in these counties compared to its limited develop- 

 ment in Ayrshire and the Lothians. 



