XXXVl. GEOLOGV OF DUMFRIESSHIRE. 



rocks in a north-westerly direction, which may probably belong to the 

 remarkable outburst of volcanic activity during the Tertiary period in 

 the Western Isles. One example is worthy of notice, as it has been 

 traced from Leadhills south-eastwards by Moffat, and across Eskdale- 

 muir by Langholm to the English border. In texture it varies from a 

 dolerite to tachylite, which is the glassy form of basalt. Another is 

 traceable for a short distance through the Permian sandstones, about a 

 mile to the north of Caerlaverock Castle. 



Throughout the County there are abundant proofs of the intense 

 glaciation which this region experienced during the glacial period, in 

 common with the rest of Scotland. By means of the striae on the rocks 

 and the transport of stones in the boulder clay, we can follow the path 

 of the ice-sheet during the extreme glaciation. The general trend in 

 Nithsdale and Annandale is towards the south-east, but on approaching 

 the shores of the Solway the ice seems to have been deflected eastwards, 

 across the belt of low ground towards Langholm and Carlisle, and 

 thence across Northumberland and down the valley of the Tyne. This 

 singular deflection was evidently due to the opposing mass of ice radi- 

 ating from the northern part of the Lake District. This easterly move- 

 ment is placed beyond doubt by the occurrence of boulders of Criff'el 

 granite in the boulder clay near Langholm and eastwards towards the 

 valley of the Tyne. The wide-spread covering of boulder clay which is 

 now found in the upland valleys, and on the low grounds, is the relic of 

 this ancient glaciation. But in the valleys draining the larger masses of 

 high ground, there are numerous moraines deposited by local glaciers, 

 of which some of the finest examples are to be found round Loch Skene, 

 at the head of Moffatdale. Moraine mounds, however, occur, far removed, 

 from any great mass of high ground, suggesting the probability that 

 local or district ice sheets may have operated in producing them. Dur- 

 ing the disappearance of the ice sheet and valley glaciers, those fluvio- 

 glacial gravels may have been formed which stretch along the basin of 

 the Annan as far as Moffat and along the valley of the Nith. The high 

 level terraces of the river valleys may have been laid down during the 

 stage of the loo feet sea beach, the remains of which are now to be found 

 bordering the Solway. Even then arctic conditions seem to have pre- 

 vailed in the South of Scotland. In the alluvia of old lakes occupying 

 hollows in the boulder clay near Edinburgh, the remains of arctic plants 

 have recently been found. Among them are dwarf birch (Betula nana), 

 willows (Salix polaris, S. herbacea, S. Reticulata), Dryas octopetala, etc., 

 together with a phylloj)od Apus glacialis, now found only in fresh water 

 lakes in Greenland and Spitzbergen. It is highly probable that some of 

 these relics of an Arctic Flora and fauna may yet be found in the old 

 lake deposits within the County. The latter stages in the geological his- 

 tory of the County are indicated by the growth of extensive peat 

 mosses, by the fifty feet and twenty-five feet sea beaches fringing the 

 Solway, and by the successive terraces of alluvium which tell us of the 

 erosion of the land by existing streams. 



