Passage Three

Many theories about the origin of the ocean have been proposed by scientists. The most widely accepted one is that the earth at some time in its very early history became hot enough to melt the materials from which it was formed. While in this molten state, lighter rock-forming materials (造岩材料) floated on the suce of the heavier ones. Then, between four and a half four billion (十亿) years ago, the molten earth cooled sufficiently to form a crust of rock that was many miles thick. Surrounding the earth was an unbroken canopy of clouds miles thick and made up mostly of water vapor. Rain falling toward the still-hot earth was heated to steam and rose to the clouds again. After many millions of years, as the earth continued to cool, its suce temperature fell below the boiling point of water. Rainwater could now remain on the earth, covering its whole suce except for the higher places on earth that had been formed from the lighter rock materials. In 1970, scientists had pieced together evidence that the lighter rock materials had formed one huge continent by a vast ocean. Then, about 200 million years ago, the great continent began to break up, the pieces moving slowly apart. The onrushing waters of the single huge ocean now entered and filled the spaces between the separating continents--and became the several oceans and seas we know today.

In the last sentence of the second paragraph, "the higher places on earth" refers to （）