SUBAEKIAL DENUDATION. 
97 
bish of a quarry which has been rejected as useless by the 
workmen, or has fallen upon the^road between the quarry 
and the building, so as to lie scattered at random over the 
ground. 
But we occasionally find in a conglomerate large rounded 
pebbles of an older conglomerate, which had previously been 
derived from a variety of different rocks. In such cases we 
are reminded that, the same materials having been used over 
and over again, it is not enough to affirm that the entire mass 
of stratified deposits in the earth’s crust affords a monument 
and measure of the denudation which has taken place, for in 
truth the quantity of matter now extant in the form of strat¬ 
ified rock represents but a fraction of the material removed 
by water and redeposited in past ages. 
Subaerial Denudation. —Denudation may be divided into 
subaerial, or the action of wind, rain, and rivers; and subma¬ 
rine, or that effected by the waves of the sea, and its tides 
and currents. With the operation of the first of these we 
are best acquainted, and it may be well to give it our first 
attention. 
Action of the Wind ,—In desert regions where no rain falls, 
or where, as in parts of the Sahara, the soil is so salt as to be 
without any covering of vegetation, clouds of dust and sand 
attest the power of the wind to cause the shifting of the un¬ 
consolidated or disintegrated rock. 
In examining volcanic countries I have been much struck 
with the great superficial changes brought about by this 
power in the course of centuries. The highest peak of Ma¬ 
deira is about 6050 feet above the sea, and consists of the 
skeleton of a volcanic cone now 250 feet high, the beds of 
which once dipped from a centre in all directions at an an¬ 
gle of more than 30°. The summit is formed of a dike of 
basalt with much olivine, fifteen feet wide, apparently the 
remains of a column of lava which once rose to the crater. 
Nearly all the scoriae of the upper part of the cone have 
been swept away, those portions only remaining which were 
hardened by the contact or proximity of the dike. While I 
was myself on this peak on January 25,1854,1 saw the wind, 
though it was not stormy weather, removing sand and dust 
derived from the decomposing scoriae. There had been frost 
in the night, and some ice was still seen in the crevices of 
the rock. 
On the highest platform of the Grand Canary, at an eleva¬ 
tion of 6000 feet, there is a cylindrical column of hard lava, 
from which the softer matter has been carried away; and 
other similar remnants of the dikes of cones of eruption at- 
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