1848.] NICOL ON RECENT FORMATIONS NEAR EDINBURGH, 23 



rare, occasionally occur. The most remarkable accumulation of them 

 is however on the Pentland Hills, that range of mountains seeming 

 to have stopped many of them in their journey to the south. Some 

 of the boulders found on these hills are remarkable for their size. 

 One angular block of mica-slate seen near Habbie's How, resting on 

 the declivity of the hill, according to a measurement I made, would 

 weigh six or eight tons. Further west I found another block, also 

 angular, of the same rock, which would weigh about three-quarters of 

 a ton. When it is considered that these masses must have been 

 carried upwards of forty miles in a direct line, floating ice seems the 

 only agent to which their transport can be ascribed. Blocks of a 

 smaller size are very common, and of a great variety of kinds, some 

 indeed of a mineralogical character, which is unlike any fixed rock I 

 have ever observed or seen described in Scotland. On one hill 1.500 

 to 1 GOO feet high, I found these travelled stones particularly abundant, 

 and apparently increasing in numbers from below upwards. In some 

 places they appeared to form as it were broad bands running nearly 

 in straight lines from N.N.W. to S.S.E., and without any reference 

 to the present declivity of the ground, except that they became more 

 numerous towards the summit of the ridge. These blocks consisted 

 chiefly of trap rocks, especially basalt, the hill on which they rested 

 being a red felspar or clay stone porphyry. Many were of sandstone, 

 sometimes rounded, but more often in angular masses, one block 

 measuring six feet long, five broad, and three feet thick, and con- 

 sequently weighing about six tons. This mass was lying on the side 

 of the hill facing the south, so that any current from the north, 

 which had propelled it thus far, would in all probability have also 

 rolled it to the bottom of the hill. 



These sandstone and trap boulders lie at an elevation above the 

 great mass of the similar rocks in the surrounding country. There 

 may be a few points of sandstone and trap at nearly the same height, 

 but only six or eight miles distant, and I do not believe that the 

 great variety of trap rocks found on the top of this hill could be col- 

 lected except from places now several hundred feet lower. This 

 therefore forms a good instance of the class of facts which Mr. Dar- 

 win's theory, lately proposed to the Society*, was intended to explain. 

 But it is also one of those cases in which that theory has particular 

 difficulties to contend with. Were the surrounding country sunk in 

 the ocean to the level of these blocks, the chain of hills on which 

 they rest would form only a few widely scattered islands in the midst 

 of a broad arm of the sea, extending from the Grampian mountains 

 on the north to the Lammermuir range on the south. The blocks 

 would thus be exposed to constant danger of being carried off" from 

 the land every time they were floated by the ice, and from the great 

 declivity of the mountain sides, would, if dropped only fifty or a 

 hundred yards from the shore, fall into water so deep that no iceberg 

 could ever again pick them up. But there is another difficulty which 

 the theory has in this case to contend with. On the flat summit of 

 one of the transition mountains to the south, whose height is usually 

 * Since published in Quart. Journ. Geol. Soc. vol. iv. pp. 315-323. 



