PERMANENCE OE CONTINENTS AND OCEANS. 119 
same drift which had been carried down to the sea at the 
beginning of the period is now undergoing a second trans¬ 
portation in the same direction. 
As when we have measured a fraction of time in an hour¬ 
glass we have only to reverse the position of our chronom¬ 
eter and we make the same sand measure over again the 
duration of a second equal period, so when the volcanic force 
has remoulded the form of a continent and the.adjoining sea- 
bottom, the same materials are made to do duty a second 
time. It is true that at each oscillation of level the solid 
rocks composing the original continent suffer some fresh 
denudation, and do not remain unimpaired like the wooden 
and glass framework of the hour-glass, still the wear and 
tear suffered by the larger area exposed to subaerial denu¬ 
dation consists either of loose drift or of sedimentary strata, 
which were thrown down in seas near the land, and subse¬ 
quently upraised, the same continents and oceanic basins re- . 
maining in existence all the while. 
From all that we know of the extreme slowness of the 
upward and downward movements which bring about even 
slight geographical changes, we may infer that it would re¬ 
quire a long succession of geological periods to cause the 
submarine and supramarine areas to change places, even if 
the ascending movements in the one region and the descend¬ 
ing in the other were continuously in one direction. But 
we have only to appeal to the structure of the Alps, where 
there are so many shallow and deep water formations of va¬ 
rious ages crowded into a limited area, to convince ourselves 
that mountain chains are the result of great oscillations of 
level. High land is not produced simply by uniform up¬ 
heaval, but by a predominance of elevatory over subsiding 
movements. Where the ocean is extremely deep it is be¬ 
cause the sinking of the bottom has been in excess, in spite 
of interruptions by upheaval. 
Yet persistent as may be the leading features of land and 
sea on the globe, they are not immutable. Some of the 
finest mud is doubtless carried to indefinite distances from 
the coast by marine currents, and we are taught by deep-sea 
dredgings that in clear water at depths equalling the height 
of the Alps organic beings may flourish, and their spoils 
slowly accumulate on the bottom. We also occasionally 
obtain evidence that submarine volcanoes are pouring out 
ashes and streams of lava in mid-ocean as well as on land 
(see Principles, vol. ii., p. 64), and that wherever mountains 
like Etna, Vesuvius, and the Canary Islands are now the site 
of eruptions, there are signs of accompanying upheaval, by 
