Features of the Earth's Surface. 347 
interior. The accompanying diagram (fig. 2) is an ideal repre- 
sentation of what must be the general character of the crust on 
this view. As before, a is the continental and 6 the oceanic 
crust. For simplicity’s sake, in all these diagrams, the crust is 
represented as spread out on a plane. If we admit that the 
general constitution of the earth is that of a solid crust cover- 
ing a liquid interior, I cannot see how the above conclusion 
can be avoided. 
assumed this all along, for there could not be a crust otherwise. 
2. The material of the crust must expand in solidifying, i. e., in 
coming crust. 3. Some portions of the crust must cool and 
thicken faster than others; these more rapidly thickening por- 
tions becoming the continents. Under these three conditions 
we nay account for continents and sea-bottoms as follows. _ 
Suppose a liquid earth consisting of heterogeneous materi 
covered with a thin crust of solid matter, cooling and the crust 
thickening everywhere by additions to its lower surface. Evi- 
dently the more conductive portions would cool and thicken 
than the less conductive portions. Thus the inequalities 
- Would commence on the lower surface, as in fig. 8. But such 
» oe 
SQ 
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of things represented by fig. 2 is assumed. <A c , 
€ same process, viz: the more rapid eens. 2 of the conti- 
nental portions a a, would cause these to rise higher and higher, 
and the ocean bottoms to sink deeper and deeper. 
us, then, by this view, the formation of continents and sea- 
bottoms, and the increase in height and size of the former, and 
in depth of the latter, is due to the unequal thickening of a float- 
9 crust by unequal cooling. 
Mountain chains.—If mountain chains were only narrow 
wrinkles on the earth's surface, we might suppose it possible 
