852 J. GEIKIE ON THE GLACIAL PHENOMENA 



the shore, we should find as we approached Beinn-nihor that the 

 striae were deflected towards the south-west, while opposite Hecla the 

 deflection would be towards the north-east. It will he remembered 

 that a deflection like this occurs in North Harris in the neighbour- 

 hood of the Cliseam. 



4. Till or Boulder-clay. — The till of South Uist is gritty and 

 earthy, with but little admixture of clay. Here and there it 

 becomes very sandy, and near the hills it is often extremely coarse, 

 being crammed with rude angular and subangular debris and 

 boulders, some of which attain a great size. Here and there the 

 stones are fairly well striated ; but in other places one has to look 

 narrowly for even one. Blunted shapes, however, are as numerous 

 as usual. At Loch Boisdale there is a good example of a " striated 

 pavement " in the till. Close to the village is a pit dug in a gritty 

 greyish-yellow and blue clay, with scattered boulders of gneiss, most 

 of which are more or less well glaciated. A number of large 

 boulders sunk in the clay, and the surfaces of which are approxi- 

 mately on the same level, are all striated across in one and the same 

 direction, W. 25° IN". 



The distribution of the till is much the same as in North Uist ; it 

 does not occur, save only in little patches, amongst the hills on the 

 east coast, but spreads over considerable areas on the low grounds to 

 the west. It has the same tendency also to gather on the lee-side of 

 roclies moutonnees and jutting crags ; and one may also trace, again 

 and again, in such sheltered places that rude dip of the stones and 

 boulders away from the protecting rock to which reference has so 

 frequently been made in my description of the till in Harris and the 

 other islands. Crumpled and contorted beds of sand and gravel 

 likewise occur in the till, pointing to the manner in which that 

 deposit was crushed and rolled forward under the ice. I also met 

 in the till with some small boulders of a dark earthy amjrgdaloidal 

 melaphyre, closely resembling some of the igneous rocks in Skye, 

 but unlike any basalt-rock I ever saw in the Outer Hebrides. At 

 Crogaire I picked out of the drift a small boulder of conglomerate, 

 which might have come from some Cambrian deposit. But these 

 were the only boulders which I was pretty sure did not belong to 

 South Uist. At the same time I think it is not improbable that 

 many of the erratics of syenite, quartz-porphyry, and other crystal- 

 line rocks which appear in these islands may really be visitors from 

 some of the Inner Hebrides, or even perhaps from the mainland. 

 At the foot of the Boisdale Hills opposite Staoleidh I saw boulders 

 of green slate which may have come from that island, where such 

 green slate and schist are well known to occur. 



5. Erratics, Moraines, and Local Glaciation. — Yery few erratics are 

 found in the low grounds of South Uist, and those that occur probably 

 belong to the till, for many show traces of glacial abrasion. Along 

 the foot of the hills, however, angular erratics are here and there 

 plentiful, one very fine example of a perched block occurring im- 

 mediately above the road a mile or more north of Airimhuilinn. 

 Here and there also one comes upon sprinklings of morainic debris 



