As the earth's matter gathered into a smaller bulk, its heat was greatly increased, so that for a time it was a hot, shining star like the sun. Gradually, however, it parted with so much of its heat that it, as we may say, froze over or became covered with a solid crust which soon became cool enough to permit the waters hitherto in the state of steam to descend upon the surface of the sphere. With this descent of the waters, which led to the formation of the seas, another stage of great importance in the history of the earth began. In the earlier ages the heat of the earth, which came from within its mass, was so great that the temperature coming from the sun was of no consequence, but when the earth acquired a crust of cold rocks, a new period began, that in which the solar heat was thereafter to be the source of most of the movements that occurred in this limited world. Thenceforward to the present day, and yet on through the ages, the sun and earth are linked together in their actions in a marvellously entangled way.
When the sun's heat began effectively to work on the earth in the manner which we now behold, the winds began to blow, the ocean waters under their influence to circulate currents, and the moisture to rise into the air to be carried to and fro and to fall as rain. It seems likely that these movements of air and water, which we know to be due to the action of the sun's heat, took place at first upon the surface which was everywhere covered by the ocean, a vast continuous sea through which the lands had not yet pierced, and in which living creatures had not begun to dwell. This universal field of waters could not have long continued, and this for the reason that certain changes in the earth itself brought about the creation of broad folds on the sea-bottom, which grew upward until dry lands rose above the level of the waters. The way in which this process took place can in general be easily understood.
After the earth had cooled to the point where its outer parts were what we term cold, and the whole of its mass approximately solid, it remained as it does to-day, exceedingly hot in its central portions, and therefore kept on slowly cooling. What we call the outer or crust part, because it had already become cool, had little heat to lose. The greater portion of the temperature, which crept away into the frigid places of the heavens, where the thermometer is always some hundred degrees below the freezing-point, came from the interior of the sphere. Because of this cooling in the deeper parts of the earth the mass shrunk in its interior portion, while the outer part, losing less heat, because it had less to lose, did not contract to anything like the same extent. Thus it came about that this crust portion which forms the surface, and that which is below to the depth of many miles, were forced to wrinkle in order to fit the diminished centre. The action may be compared, in a way, to what takes place when in an apple or other similar fruit or vegetable with a distinct skin the water dries out of the interior parts. The skin wrinkles, because it has little water to lose. Let us conceive that the heat which keeps the particles of matter apart in our earth answers to the water which separates the solid portions of the fruit, and the likeness becomes clear.