ARCHEAN AND PALEOZOIC GEOGRAPHY 445 
lation will do no harm, provided we bear in mind 
that it is mere speculation, and not fact, with which 
we are dealing. 
The Nebular Hypothesis teaches that the earth was once a 
liquid sphere, and that it has now reached a stage in its develop- 
ment when the surface is cold and the interior heated. Provided 
this hypothesis is a true explanation of the origin of the earth’s 
features, there must have been a time in its development when the 
liquid sphere became cold at the surface. So far as we have been 
abie to discover, there is no record of this period among the rocks. 
During the time when the surface of the earth, though covered 
with a thin solid crust, was still very much heated, it must have 
been enveloped by an atmosphere which contained not only all 
the gases now found in it, but also the waters of the ocean, and 
probably some substances which now exist on the surface in solid 
form. So during this period the atmosphere was very different 
from that of the present. 
In time, still following our speculation, the earth cooled so far 
that the vapor of the air began to condense and to cover the sur- 
face of the earth with a film of water. At first this heated rain, 
upon reaching the hot crust, would be evaporated and returned 
to the air in the form of steam, and this process of heated rain, 
changed back to vapor, would be passed through again and again. 
So for a time the surface received frequent baths of highly heated 
water; and this had not only the power of erosion, which water 
naturally possesses, but also the ability to do important chemi- 
eal work. This rain would obtain from the impure atmosphere 
many substances which increased its effectiveness in solution and 
change of minerals. When finally the waters of the atmosphere 
changed from the condition of vapor to that of liquid, and found 
relatively permanent rest in the form of oceans, it was still heated 
water, and in it was contained mueh rock matter in solution. As 
the warmth of the ocean decreased, some of these substances were 
