GEOLOGY OF THE OUTER HEBRIDES. 567 



pressure caused naturally a translatory movement down 

 the slope from the Harris hills to Ness. The viscous mass 

 in its slow movement ground along like a huge plough, 

 crushing the softer materials of the rocks into a " math " 

 as the grinding went on, or catching up intractable frag- 

 ments or blocks, and carrying them from their parent 

 source left them as perched blocks on hill tops, or as 

 streams of boulders on the low-lying plains. 



Ice passing down the slope of the watershed ot a 

 district behaves as snow on the roofs of houses does. The 

 local character of the glaciation of a district is demon- 

 strably seen in the County of Ross. The watershed there 

 running north and south through the middle of the 

 County sent down during glaciation times streams of 

 boulder-clay and " carried " blocks to the coast on each 

 side. Their line of march is seen pointing on the east 

 side of the N.W. and on the west to N.E. Were the 

 boulders of Lewis of as durable a character as those of 

 the mainland their longer axes would be seen pointing 

 north and south. 



But the boulders of Lewis are looser in texture, and thus 

 their forms become detached blocks sooner than the more 

 compact rocks of the same formation on the mainland. 

 Hence neither striation nor ice-scratchings are readily 

 seen in Lewis as on the mainland of the County of Ross. 

 The boulder-clay of Lewis is however such in its con- 

 stituents and formation as might have been caused wholly 

 by local agencies. 



Blown sand and marine shells buried under peat as 

 at Aignish are mixed with a felspathic clay which makes 

 excellent material for bricks. 



MINERALS. 



No minerals of commercial importance are found in 

 these Outer Islands. In the Park district of Lewis 

 according to Macculloch in his Geology of the Western 

 Isles are to be found valuable veins of serpentine and 

 of asbestos. The writer has not seen any trace of either 



