Jeffreys — Dredging among the Hebrides. 33 



unless Ms origin is much more remote than it is at present supposed 

 to be. I am not inclined to attribute the northern character of some 

 of the Hebridean moUusca to the persistence of what have been 

 called " Boreal outHers." The idea savours more of poetry than of 

 philosophy or fact. The boreal or truly arctic species whioh once 

 flourished in this district have become quite extinct, probably in 

 consequence of one of those revolutions above suggested, by which 

 the sea-bed was converted into dry land. These boreal species 

 consist chiefly of Bhynchonella psittacea, Pecten Islandicus, Astarte 

 crehricostata or depressa, Tellina calcaria, Mya truncata, var. Udde- 

 vallensis, Trochus cinereus, and Aihyris Holhollii ; and I have lately, as 

 well as on a former occasion, dredged them on the coasts of Syke and 

 "West Eoss, at depths of from 30 to 60 fathoms, or 180-360 feet. 

 They had a semi -fossilized appearance. Not one of the above-named 

 species has ever, to the best of my knowledge and belief, been found 

 in a living or recent state in any part of the British seas. All of 

 them occur in Post-tertiary or Quaternary deposits on the west 

 coast of Scotland, from a few feet above high water mark^ to 320 

 feet above the present level of the sea.^ The greatest subaerial 

 height (320 feet) being added to the greatest submarine depths, as 

 above (360 feet), gives an extent of elevation and subsidence 

 equal to 680 feet. But as Pecten Islandicus, for example, now 

 inhabits the arctic ocean at depths varymg from 5 to 150 fathoms, 

 let us take the average of these depths — -viz., 77| fathoms, or 465 

 feet, and add it to the 680 feet. This would make 1145 feet, and pro- 

 bably represent the height at which the sea-level might be supposed 

 to have stood when P. Islandicus lived on the highest fossiliferous 

 spot, noticed by Mr. Watson. The non-fossiliferous Boiilder-clay, 

 indicating the simultaneous presence of arctic land, which was also 

 subject to glacial conditions, is stated by Mr. Watson^ to be about 

 800 feet higher than the marine deposit. The height of the layer 

 of sea-shells on Moel Tryfaen, in Carnarvonshire (evidently the 

 remains of an ancient beach) exceeds that of the similar deposits 

 at Cardigan by more than 1300 feet ; and the difierence of height 

 observed in the case of other fossiliferous deposits in the north of 

 England {e.g. Manchester and Kelsey Hill) shows that the disturbing 

 movement has been unequal and probably not synchronous over the 

 same area. It would seem that the extent of such oscillation has 

 not altogether amounted to 2000 feet in the British Isles, taking 

 Moel Tryfaen as the greatest height, and the Shetland sea-bed as 

 the greatest depth, at which quaternary shells of recent species occur. 

 The Scotch and Irish deposits, however, are on the whole far more 

 ancient than those of Wales and England, judging from their geo- 

 graphical nature ; the former are chiefly arctic, and the latter merely 

 northern. Whether other parts of the North Atlantic sea-bed have 



^ British Association Eeport, 1862, Trans. Sect. p. 73; Jeffreys on an Ancient Sea- 

 ted and Beach near Fort AYilliam, Inverness-shire. 



2 Transactions of the Royal Society of Edinburgli, 1864, p. 526 ; Eey. E. B. Watson 

 on the Great Drift-beds with Shells in the South of Arran. 



3 Lc. pp. 524. 



VOL. IV. — NO. XXXI. 3 



