1 88 PREHISTORIC EUROPE. 



the meeting of the mers de glace of the Highlands and Southern 

 Uplands. A much more striking example of this kind of deflec- 

 tion remains to he mentioned. In Caithness the boulder-clay 

 has yielded many broken sea-shells, not a few of which are 

 finely striated. Perfect shells are rarely met with. The broken 

 fragments are scattered about in precisely the same manner 

 as the stones, and they belong to a heterogeneous mixture of 

 arctic, boreal, and southern forms. 1 For a long time the origin 

 of this shelly clay was a puzzle, but the solution of the puzzle 

 was at last furnished by my colleague Dr. Croll, 2 who pointed 

 out that the clay was the bottom-moraine of a trier de glace which 

 had overflowed Caithness from south-east tc north-west, to do 

 which it must first have traversed the Moray Firth, and hence 

 came the shelly ddbris and certain stones that, so far as we know, 

 could have been derived from no other direction. This bold 

 suggestion met with considerable opposition when it was first 

 made, for it involved a most remarkable conclusion. Dr. Croll 

 showed that the ice which overflowed Caithness had been 

 deflected out of its normal path by the presence of another 

 immense mer de glace flowing outwards from Scandinavia, and 

 he further maintained that the Islands of Orkney and Shetland, 

 when they came to be thoroughly examined, would prove to be 

 striated from east to west. This conclusion has been subse- 

 quently borne out by the observations of my brother, Professor 

 Geikie, and Mr. B. N. Peach, in Caithness, 3 and by a detailed 

 examination of the Shetlands 4 by the latter in company with 

 Mr. J. Home, and similar results have been obtained by the 

 same geologists in Orkney. Not only are the Shetlands striated 

 across in a general east and west direction, but the till covering 

 the western part of the islands is crammed with stones derived 

 from the east. My friend, Mr. Amund Helland, of Christiania, 

 has also visited these islands, and confirmed the observations 

 made by my colleagues. 5 The striae and the carry of the 



1 See Quart. Journ. Geol. Soc. (Jamieson), 1866, p. 261. 



2 Geol. Mag., vol. vii. p. 209. 3 Great Ice Age, p. 179. 

 4 Quart. Journ. Geol. Soc.,r. xxxv. p. 778. 



6 Zeitsehr. deutsch. geol. Ges., Bd. xxxi. (1879), p. 63. 



