4 
AMERICA THE OLD WORLD. 
was the resistance they encountered, that some 
portions of the earlier rock-deposits are perfo- 
rated with numerous chimneys, narrow tunnels 
as it were, bored by the liquid masses that poured 
out through them and greatly modified their first 
condition. 
The question at once suggests itself, How was 
even this thin crust formed ? what should cause 
any solid envelope, however slight and filmy 
when compared to the whole bulk of the globe, 
to form upon the surface of such a liquid mass ? 
At this point of the investigation the geologist 
must appeal to the astronomer ; for in this vague 
and nebulous border-land, where the very rocks 
lose their outlines and flow into each other, 
not yet specialized into definite forms and sub- 
stances. — there the two sciences meet. Astron- 
omy shows us our planet thrown off from the 
central mass of which it once formed a part, to 
move henceforth in an independent orbit of its 
own. That orbit, it tells us, passed through 
celestial spaces cold enough to chill this heated 
globe, and of course to consolidate it externally. 
We know, from the action of similar causes on a 
smaller scale and on comparatively insignificant 
objects immediately about us, what must have 
been the effect of this cooling process upon the 
heated mass of the globe. All substances when 
heated occupy more space than they do when 
