262 THE EVOLUTION OF CONTINENTS. 
substance, yet in a general way, it is true that the 
heavier substances were found below, and the lighter 
near the surface. As the temperature fell, the more in- 
fusible substances solidified first, and, as these, as a 
class, are lighter than, for example, the iron which with 
other materials made up the magma, they rose as a 
scum, or crust, to the top, a prosess which we see re- 
peated on a minute scale, at every smelting furnace, in 
the slag which floats on the molten metal. 
The slag or crust, formed on the lava would gradually 
increase in thickness very much as the ice increases on 
the surface of a pond. Twenty, thirty, or a hundred 
miles would not be very thick for such a formation. 
Such a process, if undisturbed, would have covered the 
earth uniformly — substantially so—with a crust of 
present rock-materials,— very much as ice covers, and 
floats on, the surface of a pond. 
As the interior of the globe continued to lose heat, 
consequent contraction went on, which tended to pro- 
duce elevations and depressions in the floating crust. 
wrinkles on its surface with as little system as those on 
a withered apple. If undisturbed, such might now be 
its condition ; but other forces were at work tending to 
break up the crust, and thus to prepare it for rearrange- 
ment in one or more masses. There were the lunar 
tides, and in a less degree the solar tides. These in seas 
thousands of miles in depth, must have been much 
greater than ocean tides now. ‘Their effect was to 
weaken, by frequent flexure, the crust, and, to a certain 
degree, to cause a relative movement westward. There 
were also enormous eruptions of vapors and gases such 
as those now occurring in the sun. And there were seis- 
mic forces, probably incomparably greater than those 
now in operation. To these we may with probability 
add the influence of cyclones which in such an atmos- 
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