312 Dr Ch. Martins on the Marks of Glacial Action 



phenomena connected with which cannot otherwise be ex- 

 plained than by the action of a glacier. This remarkable 

 rock occurs at the foot of the southern face of Blackford 

 Hill, on the bank of the streamlet called Braid's Burn. The 

 rock, which is phonolitic, overhangs at an angle of 40'', 

 forming a recess or cave ten feet deep, and the roof or over- 

 hanging surface of this recess is polished and striated for a 

 distance of thirty feet. The streaks run from WNW. to ESE., 

 that is, in the direction of the valley, and in many places may 

 be perceived the quartzoic sand which has produced the striae 

 still adhering to the rock. In Switzerland we have frequent 

 opportunities of demonstrating, on the margins of the glaciers, 

 that the sloping sides are striated /rom beneath by the plas- 

 tic mass of the glacier, which forces itself under the rock, 

 and polishes its inferior surface ; but how are we to explain 

 a similar fact by the action of floating icebergs 1 These not 

 being able to carry pebbles, except at their lower surface, 

 we must needs suppose that Blackford Hill had been striated 

 by floating icebergs, which had been capsized in such a man- 

 ner that the lower surface had become the superior. It 

 must, moreover, be admitted that these ice-blocks were so 

 situated under this rock, which at this period must have 

 been at the bottom of the ocean, and possibly 1200 feet be- 

 low its surface. And all this admitted, the difficulty is not 

 even yet removed ; for if the imagination conceives the pos- 

 sibility of a floating iceberg grazing the rocks by pressing on 

 them with all its weight, it cannot comprehend how this ice 

 striates by its upper surface, or by its side, whilst it passes 

 under a rock conveyed by a current. Mechanical agency 

 and experience alike teach us, that the most trifling obstacle 

 stops or diverts a floating iceberg, and that the impulsion which 

 it receives in a current will never be sufficiently powerful to 

 enable it to streak a rock so hard as the one at Blackford Hill. 

 The freestones and traps in the neighbourhood of Edin- 

 burgh having been striated by glaciers, these must have 

 descended from mountains ; and as a matter of fact, the re- 

 searches of Scottish glacialists have discovered specimens in 

 most of the valleys. Mr Charles Maclaren and Mr Smith of 

 Jordanhill having requested me to visit that of the Gare 



