A. Milward, Esq. on the Motion of a Lava-Stream. 47 
I did not attempt to explain the origin of the ridges, as I 
was not then aware that they had been observed.* 
It is curious, yet at the same time perfectly consistent, 
that, while in the mud-slide, the intervals between the ridges 
are most compact in structure, in the glacier the contrary is 
the case. In the mud-slide, the water drains away from the 
ridges the finer particles, and deposits them—a sort of allu- 
vium—in the hollows. The difference of consistency is thus an 
after phenomenon. In the glacier, there can be no such after 
phenomenon. In this case, the cause is I think dependent 
upon the varying action of the phenomena concerned in the 
formation of the glacier, rather than upon a mechanical action. 
Professor Forbes illustrates in an admirable manner, the 
way in which pressure from behind causes a wave or ridge in 
advance, in consequence of the posterior parts finding less 
obstruction in rising and over-riding what is in front of them, 
than the anterior parts do in forcing their way forwards. But 
I think, in the instance of the glacier, the ridge owes its 
origin chiefly to the accumulation at the foot of the steeps of 
névé, as I ventured to suggest in my former paper. My 
reason for this opinion is as follows: 
Upon the ground which Professor Forbes so well explains, 
we may expect in all cases to find a ridge, formed by pressure, 
at a certain distance from the foot of the névé slopes. So 
long as these slopes remain unchanged in character, the 
form of the ridge will be permanent, although the névé or 
ice of which it is composed constantly moves on. On the oc- 
currence of summer gushes, this ridge will necessarily be en- 
larged, because the weight or force which causes it, is in- 
creased ; and while it remains enlarged, the part of the gla- 
* I owe an apology to Professor Forbes, for not being aware that he had 
described the ridges in his Fifth Letter. Ill health, of long continuance, obliges 
me to be absent from England a great part of the year; so that, withdrawn 
as I am from the means of reference, I cannot pretend to keep pace with the 
information published on any subject, and can only be excused for intruding 
on public attention when the “accident of travel” enables me to impart a 
knowledge of facts which would otherwise pass unnoticed. In the present in- 
stance, the edition of Professor Forbes’s Travels which I used was the jirst, which 
does not contain the Fifth Letter, reprinted from this Journal. 
