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POPULAR SCIENCE REVIEW. 
as much as possible, by the aid of geological maps, the evidence 
of a trusty guide, and above all by personal observation, what 
the rocks consist of which overhang a glacier at different 
localities along its course. For example, the great Aletsch 
glacier is more than twenty miles in length, and in its mo- 
raines are rocks of totally different formations: some are 
wanderers from the source of the glacier among the great 
snow-fields ; others are derived from rocks in situ nearer 
to the foot of the glacier at the Bel Alp. Again, at the Zmutt 
Glacier near Zermatt, a rock fragment which has fallen from 
the Matterhorn is a volcanic rock, and differs in toto from a 
fragment from the Dent Blanche. In many instances, and many 
localities, these ice-borne masses render evidence that the ice of 
existing glaciers once rose far above their present surface, and, 
stranded high upon mountain flanks, sometimes a thousand feet 
above the glacier, or the site of a glacier long since melted away, 
it is often of importance to the geologist to know the home of 
the parent rock from which this or that wanderer has been 
derived. This is especially the case in hilly countries, where 
much debris has fallen from the rocks into the mountain vales. 
More than once it has occurred to me to be sent to see old 
glacier moraines which were no moraines at all, and which 
would not have happened had my friend known the difference 
between atmospheric debris and the erratic masses which more 
or less are generally to be traced in true moraines. It is well 
also to accustom the eye to recognise another witness of the 
existence of a glacier — viz. the “ roche moutonee ,” so called 
from the supposed similarity to the round and smooth outline 
of a sheep’s back when newly sheared and when lying down. 
These roches moutonees are portions of the rock, in situ , over 
which a glacier has passed, and owe their round and smooth 
outline to the action of the ice continually grinding over them 
and planing off the inequalities. Blocs perches also are rock 
masses which have been carried upon a glacier often for 
long distances, and left perched, on the melting and retiring 
of the ice, far from the parent rocks from which they 
were derived. Among the Mont Blanc Alps are rocks of 
different character. There are granite rocks, gneiss rocks, con- 
glomerates and various others ; and it is very striking to see a 
bloc perche of Mont Blanc granite, glistening white in the sun, 
perched upon some dark rock on the side of a hill, far from the 
noble mountains from which it has travelled. 
Some years have passed away now since I accompanied my 
friend Sir William Guise, who also loves the lore of the rocks, to 
examine glacial phenomena among some of the noblest of Alpine 
scenery, to collect Alpine minerals and fossils, Alpine plants, 
Alpine butterflies, and Alpine everything. We first visited the 
