CHAP. xiii. SUBAQUEOUS DRIFT IN PERTHSHIRE. 245 



fectly horizontal strata, the lowest portion of them 300 feet 

 above the river and 600 feet above the sea. From this 

 elevation to an altitude of nearly 1200 feet the same series 

 of strata is traceable, continuously, up the slope of the moun- 

 tain, and some patches are seen here and there even as high as 

 1550 feet above the sea. They are made up in great part of 

 finely laminated silt, alternating with coarser materials, through 

 which stones from four to five feet in length are scattered. 

 These large boulders, and some smaller ones, are polished on 

 one or more sides, and marked with glacial strias. The sub- 

 jacent rocks, also, of gneiss, mica slate, and quartz, are every- 

 where grooved and polished as if by the passage of a 

 glacier.* 



At one spot a vertical thickness of 130 feet of this series 

 of strata is exposed to view by a mountain torrent, and in all 

 more than 2000 layers of clay, sand, and gravel were counted, 

 the whole evidently accumulated under water. Some beds 

 consist of an impalpable mud-like putty, apparently derived 

 from the grinding down of felsjoar, and resembling the mud 

 produced bj^ the grinding action of modern glaciers. 



Mr. Jamieson, when he first gave an account of this drift, 

 inferred, in spite of the absence of marine shells, that it 

 implied the submergence of Scotland beneath the ocean after 

 the commencement of the glacial period, or after the era of 

 continental ice indicated by the subjacent floor of polished 

 and grooved rock. This conclusion would require a submer- 

 gence of the land as far up as 1550 feet above the present 

 sea-level, after which a great re-upheaval must have occurred. 

 But the same author, having lately revisited the valley of the 

 Tummel, suggests another possible, and I think probable, 

 explanation of the same phenomena. The stratified drift in 

 question is situated in a deep depression between two but- 

 tresses of rock, and if an enormous glacier be supposed to 



* Jamieson, Geological Quarterly Journal, vol. xvi. p. 360. 



