RAMBLES OP A GEOLOGIST. 263 



perhaps in the middle of the open bay formed by the pro- 

 montory on which it rests, for the small proportion of other 

 than Liasic materials which it contains serves to show that it 

 could be derived from no great distance, an outlier of the 

 Lower Lias. The icebergs of the cold glacial period, pro- 

 pelled along the submerged land by some arctic current, or 

 caught up by the gulf-stream, gradually grated it down, as a 

 mason's labourer grates down the surface of the sandstone 

 slab which he is engaged in polishing ; and the comminuted 

 debris, borne eastwards by the current, was cast down here. 

 It has been stated that no Liasic remains have been found in 

 the boulder-clays of Scotland. They are certainly rare in 

 the boulder-clays of the northern shores of the Moray Frith ; 

 for there the nearest Lias, bearing in a western direction from 

 the clay, is that of Applecross, on the other side of the island ; 

 and the materials of the boulder-deposits of the north have 

 invariably been derived in the line, westerly in its general 

 bearing, of the grooves and scratches of the iceberg era. But 

 on the southern shore of the frith, where that westerly line 

 passed athwart the Liasic beds of our eastern coast, organisms 

 of the Lias are comparatively common in the boulder-clays ; 

 and here, at Blackpots, we find an extensive deposit of the 

 same period formed of Liasic materials almost exclusively. 

 Fragments of still more modern rocks occur in the boulder- 

 clays of Caithness. My friend Mr Robert Dick of Thurso, 

 to whose persevering labours and interesting discoveries in 

 the Old Red Sandstone of his locality I have had frequent 

 occasion to refer, has detected in a blue boulder-clay, scooped 

 into precipitous banks by the river Thorsa, fragments both 

 of chalk-flints and a characteristic conglomerate of the Oolite. 

 He has, besides, found it mottled from top to bottom, a full 

 hundred feet over the sea-level, and about two miles inland, 

 with comminuted fragments of existing shells. But of this 

 more anon. 



