James Croll — Boulder-clay of Caithness. 209 



weather in nearly all cases to a pale yellow tinge. These beds 

 are exposed along the Tweed, from the junction of that river with 

 the Ettrick, almost to the town of Melrose, wherever the bank or 

 bed of the river is rocky. ,,,. 



The fossils here present a more decided Coniston facies, and include 

 G. Sedgwiclcii — spinigerus (Nich.), proteus (Barr), and triangulatus 

 (Hark.) — G. lohiferus (M'Coy), G. exiguus (Nich.), G. Nilssoni, O. 

 priodon (Bronn), G. colonus (Barr), G. turriculatus (Barr), and 

 Betiolites Geinitzianus (Barr), together with Dictyonema, Peltocaris 

 aptychoides (Salt.), Ceratiocaris sp., Orihoceras, and many indeter- 

 minate fossils, among which are fragments of a couple of varieties 

 of Corals, a reticulated fossil — perhaps Stromatopora striatella 

 (Lonsdale) — and what Dr. Page considers to be the jaw-foot of some 

 species of Pterygotus. 



(2.) The Gala Grits. — Overlying the Abbotsford beds, and divided 

 from them by a persistent sett of laminated or tiley beds (the Bridgend 

 Flags), and also surmounted by another band of nearly the same 

 nature (the Ellwand Slate), come the Gala Grits. They consist 

 generally of heavy grey wackes, with shaly partings, and contain 

 occasionally seams or beds of foliated shale of varying colours, 

 generally yellow or reddish, but sometimes of a cold green colour. 

 The thinner seams are finer in texture, often grey and crumbly, 

 sometimes blue or dark green, and hard and splintery. The grits of 

 the lower beds are fine-grained, homogeneous, green, and thin-bedded ; 

 those of the upper portions are heavier, of a fine blue colour, and 

 very quartzose. 



Fossils are comparatively rare, aijd where found are those of the 

 Abbotsford Greywackes. 



{To be concluded in the June Number.) 



V. — The Bouldek-clat of Caithness a Product of Land -ice. 



By James Croll, of the Geological Survey of Scotland. 



PART I.i 



The "Nature of the Caithness Boulder-clay. 



A CONSIDERABLE amount of difficulty has been felt by geolo- 

 JljL gists in accounting for the origin of the Boulder-clay of 

 Caithness. It is an unstratified clay, of a deep grey or slaty 

 colour, resembling much that of the Caithness flags on which it 

 rests. It is thus described by Mr.. Jamieson (Quart. Jour. Geol. Soo; 

 vol. xxii. p. 261) : 



" The glacial drift of Caithness is particularly interesting as an 

 example of a Boulder-clay, which in its mode of accumulation and 

 ice-scratched debris very much resembles that unstratified stony 

 mud which occurs underneath glaciers — the ' moraine profonde,' as 

 some call it. 



^ This paper is illustrated by a Map (Plate X.), wliich will appear, together 

 ■with the concluding part of the article, in our June iS^ umber. — Edit. > 



vol. VU. — NO. LXXI. 14: 



