272 TEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Eeb. 7, 



wliich prevails thence all along the east coast as far south as Fife- 

 shire. In the neighbourhood of Peterhead, as for example, at the 

 Invemettie brickwork (See Quart. Journ. Geol. Soc. vol. xiv. p. 518, 

 1858), there are many boulders of red and grey sandstone, and also 

 of a tough greenish-coloured stone, all of which resemble rocks that 

 occur in Caithness, but, so far as I know, not in the adjoining parts 

 of Aberdeenshire. I have always been at a loss to account for these 

 boulders near Peterhead, and also for the dark -grey tint of the clay 

 between that town and Praserburgh, seeing that the rocks of the 

 district consist of mica-slate and granite. But looking at the direc- 

 tion of the glacial markings, T am somewhat inclined to think that 

 both the boulders and the dark muddy sediment in this low projecting 

 corner of Aberdeenshire may have drifted from Caithness. 



8. Place of the Caithness Drift in the history of the Glacial period. 

 — I have now to consider the relation in time of the glacial pheno- 

 mena I have been describing to those of the rest of Scotland. In 

 the parts of Caithness which I examined, it seems to me that we 

 have only one glacial deposit, and I am further of opinion that we 

 cannot separate, in point of time, the period of its accumulation and 

 present arrangement from that of the scratching of the rocks on 

 which it lies : for the coincidence in direction of the scores on the 

 rocks with those on the stones imbedded in the drift shows that it 

 was one great movement — long- continued probably — that marked 

 the rocks and carried along the mass of stony mud that now rests 

 upon them ; in fact it was the movement of the drift across the 

 surface of the rocks that scratched them. Now in the midland 

 region of Scotland we have evidence of three well-marked stages in 

 the Glacial period — 1st, the great glaciation of the surface and 

 deposition of the Old Boulder-clay or till ; 2nd, the finely laminated 

 glacial-marine beds ; 3rd, the overlying gravels and moraines. To 

 which of the divisions represented by these three are we to refer the 

 Caithness drift ? or is it the equivalent of any two of them, or of all 

 the three ? 



The Caithness Drift contains remains of sea-shells all through it, 

 often from top to bottom, and these shells are broken, rubbed, and 

 scratched, evidently by the same agency that marked the rocks and 

 boulders. This is an important fact, for it gives us a date for the 

 action. The scratching and breaking of the shells was an event at 

 least as late as the time when the MoUusca lived that formed the 

 shells ; and, seeing that the shells extend to the bottom of all the 

 sections of the drift, it is further evident that none of it was lodged 

 in its present position at a more remote date. Indeed, it is in the 

 lower part of the sections that the fragments of shells chiefly occur; 

 in some cases, as for example at Wick and Keiss, they are scarcely 

 to be found in the upper portion. If, then, we could find the date 

 of the shells, we should have a clue to the age of the drift itself. 

 In the midland region of Scotland the Old Boulder-clay, and the 

 scratching of the ice-worn surface of rock on which it rests, evidently 

 preceded the time when the moUusca lived whose remains we find 

 in the marine beds above it. At Loch Gilphead, in Argyleshire, 



