240 Proceedings of the Royal Physical Society. 



The Eskdale dyke has thus more than merely local interest. 

 It forms one of a vast series of fissures up which, during 

 Tertiary time, lava has risen, and its careful study may do 

 something towards the elucidation of the mechanism of the 

 eruptions to which it owes its origin. 



The north-western - limit to which this dyke has been 

 traced may not be its actual termination in that quarter. A 

 short way further north several similar dykes appear and 

 continue in the same direction for many miles towards the 

 north-west. It is not always possible to find continuous 

 evidence of the presence of dykes at the surface, partly of 

 course from the depth of soil or superficial drifts, but partly 

 also, no doubt, because a dyke, though continuous at some 

 depth below, did not everywhere rise equally near the sur- 

 face. The remarkable series of dykes which runs through the 

 high ground round the sources of the Tweed, Annan, and 

 Clyde may therefore be parts of one subterranean mass of 

 igneous material which, running on towards the north-west, 

 reappears in great force at the surface along the shores of the 

 Firth of Clyde. The coast-line from Ardrossan to Gourock is 

 traversed by many large and small veins of basalt. The con- 

 vergence of the dykes in the counties of Ayr, Eenfrew, Dum- 

 barton, Stirling, and Lanark towards the angle of the Clyde 

 estuary, opposite the mouth of Loch Long, is remarkable. 



From where it first appears among the high ground to the 

 north of the village of Leadhills, the Eskdalemuir dyke runs 

 in a tolerably straight course towards the south-east, across 

 the range of the Lowther Hills, the valley of the Powtrail 

 Water, and the high grounds surrounding the upper part of 

 the valley of the Clyde, until it descends into the broad plain 

 of Annandale below Moffat. Here it is lost under the alluvial 

 floor of the valley, but it reappears immediately among the 

 slopes on the east side, whence it pursues a persistent though 

 somewhat tortuous course across hill and dale until it reaches 

 the Esk, where Jameson's observations were made. Crossing 

 this river, it keeps along the line of the valley as far as 

 Langholm, where, mounting up into the moors drained by 

 the Tinnis Water, it finds its way across into the dale of 

 the Liddel, and passes thence into England. How much 



