1878.: 
Notices of Books. 
121 
are obviously unable to roll or slide down. How, then, is the 
effect produced ? The beds of quartzite vary greatly in hard- 
ness ; some crumbling readily away, whilst others scarcely yield 
at all to ordinary weathering. The softer portions being thus 
worn away, the harder beds are left as long projecting ridges 
along the crests and flanks of the hill ranges, and ultimately, being 
deprived of their supports, give way, and the fragments fall over 
upon the sloping hill-sides when they become imbedded in vege- 
tation, a slight inequality in the surface of the turf merely indi- 
cating the blocks buried beneath it. From a variety of causes, 
the whole soil-cap, blocks included, gradually creeps down even 
very gentle slopes. Thus the alternate expansion and contraction 
of the vegetable mass, according as it is soaked with water or 
comparatively dry, gradually pushes the blocks downwards. 
The rain-water as it runs down clears away the movable matter 
before them, and the vegetable mould on which they rest “ is 
undergoing a perpetual process of interstitial decay and removal.” 
In this manner the blocks are gradually swept down the slope 
and collected in the valley. 
The author gives analogous cases, though on a less striking 
scale, which he has observed in the West Highlands of Scotland 
and elsewhere. He adds : — “ It seems to me almost self-evi- 
dent that wherever there is a slope, be it ever so gentle, the soil- 
cap must be in motion, be it ever so slow ; and that it is drag- 
ging over the surface of the rock beneath the blocks and boulders 
which may be imbedded in it ; and frequently piling these in 
moraine-like masses where the progress of the earth-glacier is 
particularly arrested, as at the contracted mouth of a valley 
where the water percolating through among them in time re- 
moves the intervening soil. As the avalanche is the catastrophe 
of ice-movement, so the land-slip is the catastrophe of the 
movement of the soil-cap. As I have already said, I should be 
the last to undervalue the action of ice, or to doubt the abun- 
dant evidences of glacial adtion ; but of this I feel convinced, 
that too little attention has been hitherto given to this parallel 
series of phenomena, which in many cases it will be found very 
difficult to discriminate ; and that these phenomena must be 
carefully distinguished and discriminated before we can fully 
accept the grooving of rocks and the accumulation of moraines 
as complete evidence of a former existence of glacial conditions.” 
The theory thus suggested evidently demands very careful ex- 
amination. Every geologist must have observed blocks embedded 
in vegetable mould at considerable distances from the rocks whence 
they were obviously derived upon slopes too gentle to admit of 
rolling or sliding and in situations where the accumulation of a 
head of water sufficient to bear them down seems utterly impro- 
bable. If they have travelled since these slopes can have been 
glaciated, if they are still travelling, Sir W. Thomson’s theory 
must be accepted. But the movement is exceedingly slow, so as 
