822 J. GEIKTE ON THE GLACIAL PHENOMENA 



the ice which overflowed Suainabhal did not come from the high 

 grounds lying to the west and south, but from the south-east. In 

 other words, it was not of local origin, hut formed part of a general 

 glacial covering that flowed in one determinate direction (S.E. to 

 JST.W.) across the whole breadth of Lewis ; and we gain some idea 

 of its thickness from the fact that no local glaciers which mayhave 

 come from the mountains of North Harris were of sufficient extent 

 to deflect it. It rose high upon the northern slopes of these moun- 

 tains, and Suainabhal (1300 ft.) was simply overwhelmed, like a 

 small boulder in the bed of a stream. 



3. Shelly Boulder-clays, Sfc. — Along the sea-coast, near the Butt 

 of Lewis, at Traigh Chrois, namely, and between that bay and the 

 point called Sinntean, some excellent sections of Boulder-clay are 

 exposed. Similar exposures also occur in the sea-cliffs of Traigh 

 Chealagbhat, close to the Port of Ness. These sections were first 

 noticed by Macculloch, whose description, however, is very meagre*. 

 He speaks of the deposits as " alluvial matter," but does not attempt 

 to account for their anomalous position, merely remarking that they 

 seem to owe their origin to distant changes, on which he has " no 

 other conjecture to offer than such as have been often produced to 

 account for similar alluvia in various parts of the globe." 



I have given a somewhat condensed account of these deposits 

 elsewhere t, but describe them here a little more particularly, that 

 glacialists may be able to compare them with similar deposits in 

 other parts of Britain. The sections show a succession of three 

 well-marked divisions, viz. : — 



a. Lower shelly Boulder-clay. 



b. Interglacial beds. 



c. Upper shelly Boulder-clay. 



But these are not always present in one and the same section. 

 In some places the Lower Boulder-clay is wanting, and the inter- 

 glacial beds vary much both in thickness and composition. 



a. Lower shelly Boulder -clay . — This is a dark greyish-brown, 

 sandy or earthy clay, which is usually tough, but when wetted 

 becomes a soft unctuous silt. Although closely resembling similar 

 deposits in the Lowlands of Scotland, it yet differs very markedly 

 from the till which is commonly met with throughout Lewis and 

 the islands of the Outer Hebrides generally. It is quite unstrati- 

 fied, but here and there contains irregular lenticular patches, veins, 

 and beds of gravel, sand, and silt or clay. The included stones 

 are generally blunted and subangular, and now and again show 

 well-marked striae. They vary in size from mere grit up to blocks 

 several feet in diameter, and consist chiefly of gneissose rocks. But 

 boulders of red sandstone, conglomerate, and quartz-rock, and liver- 

 coloured quartz-pebbles, which have evidently come from the Cam- 

 brian conglomerates, occur somewhat numerously in places. Here 



* ' Description of the Western Islands of Scotland,' vol. i. p. 189. 

 t ' Great Ice Age,' 2nd edit. p. 168. 



