OE THE OUTER HEBRIDES. 863 



ice set along the inner margin of the Long Island, one to south- 

 west, the other to north-east. Now it is evident that these currents 

 would eventually sweep round the obstruction caused by the great 

 Hebridean ridge, and thereafter continue in the general direction of 

 the ice-now. If we examine the Admiralty charts of the Minch, we 

 shall find that the ridge in question presents a steep face to the east, 

 from a point opposite the Eye Peninsula to another point which lies 

 15 miles south of Barra Head. North of Eye Peninsula the sea- 

 bottom rises with a gentle gradient to the coast of Lewis, so that 

 there was nothing here to prevent the north-east under-current 

 creeping up and over the low ground near the Butt. Let us note 

 further that the most northerly rock-basin of any importance along 

 the inner margin of the Long Island occurs just opposite the Eye 

 Peninsula. Beyond this point the under-current met with little or 

 no obstruction, and it ceased therefore to exert the same degree of 

 erosion upon the sea-bottom. It now gradually turned away north 

 and north-west, following the trend of the ridge, until — always 

 creeping more and more to the west — it at last overflowed the 

 northern end of Lewis. As its course lay along what had only 

 recently been the sea-bottom, its moraine profonde necessarily con- 

 tained shells and other marine exuvise, and a much more consider- 

 able admixture of silt and clay than the unfossiliferous till through- 

 out the rest of the Outer Hebrides. 



No shelly boulder-clay occurs at the southern termination of the 

 Long Island. But a glance at the Admiralty charts at once explains 

 its absence. The steep ridge of rock that opposed the passage of 

 the mer de glace continues, as I have said, to a point 15 miles south 

 of Barra Head. Here the great Hebridean ridge terminates, and 

 here the under-current of the ice was first enabled to change its 

 direction and flow westward, its course being mapped out on the 

 sea-bottom by the broad hollows it scooped out. It is evident, 

 therefore, that, as the ice at the bottom of the Minch could not 

 invade Barra and the islands to the south, no shelly boulder-clay 

 would be pushed over any part of the ridge that now remains above 

 water. 



The interglacial deposits indicate a considerable change of climate ; 

 the ice-sheet which had overflowed the Long Island had now 

 vanished, so far as the Outer Hebrides were concerned, and the sea 

 once more occupied the Minch and rose to a height of 175 feet at 

 least above its present level. The actual limits of submergence we 

 cannot tell, for the reasons that follow ; neither can we make any 

 definite statement as to the character of the interglacial climate. 

 The deposits are mere fragments, and their full meaning will 

 probably not be realized until they are viewed in connexion with 

 what is known of interglacial deposits in other regions. 



In the Upper shelly Boulder-clay we read the return of precisely 

 the same conditions as those that witnessed the accumulation of the 

 Lower. The ice-sheet again overflowed the Long Island, and its 

 under-current stole over the Butt as before. The interglacial beds 

 were now subjected to much denudation. "We can hardly doubt 



