“Then I should think the oceans would be growing deeper,” ventured Pedro.
“Right again. When this earth reaches its old age,—speaking in terms of centuries,—it will likely be all ocean. And there used to be far more land, in proportion, than there is now. There was less ocean water then because of all that is continually pouring through hot springs.
“Of course the land is slowly being washed back into the ocean. And the higher the mountains, the steeper the stream beds, and hence the faster the streams, and the faster they erode the high elevations, till finally all is reduced to sea level again.”
“Then how do the mountains get rebuilt?” Pedro testified his interest.
“The earth has, as I think I said before, shrunk between 200 and 400 miles in diameter,—since the beginning,—‘when the earth was unformed and void, and darkness was upon the face of the deep.’ It is still shrinking. And this internal movement is felt on the surface in differences that generally amount to only a few hundred feet. I can show you places over there on the East wall of the Sierras where the mountains have been upthrust that way.
“Then, every now and again, the interior activities fairly break the rocky earth shell or lithosphere, and whole mountain ranges are raised. There have been at least eight such minor breaks in the earth crust in North America alone, and each time ranges perhaps a thousand miles long, or more, have been raised near one end of the continent or the other. In addition, there have been major re-adjustments that thrust whole continents higher and ocean beds lower. Geologists find evidence of at least six of these major breaks in the earth crust,—marking the beginnings of the Archeozoic Era, when life originated, the Proterozoic Era, or age of invertebrates, the Paleozoic Era or age of fish dominance, the Mesozoic Era or age of reptile dominance, the Cenozoic Era or age of mammal dominance, and the present Psychozoic Era or age of man.”
“Phew!” whistled Long Lester again. “Don’t tell me this earth used to be all fish.”
“It did, though. We’ll go into that some other time. I’ll just finish about continent building now, and then we’ll turn in. At these times when the lands are at their highest and the oceans are smallest in breadth, (because greatest in depth), the continents are united by land-bridges such as those we have now uniting North and South America.”
“And Alaska and Asia?” suggested Ted.
“Practically, yes. And probably, at one time, South America and Australia. These land-bridges changed the direction of the ocean streams. You know in the age of reptiles there was nothing to divide the Atlantic from the Pacific. Added to that, the high mountain ranges took the moisture out of the winds from the oceans, as the Rockies now do the Pacific trade winds, so that by the time they reach Nevada there is no moisture left in them to form clouds and fall in rain, and we have desert.