Part 16 (1/2)

”Either that or volcanic lava.”

”But how did it----”

”Just a minute. Of course land ma.s.ses have gone down as well as up, but the general trend has been decidedly upward, while the trend of the ocean floor has been downward. At that, the sh.e.l.l of the earth--so to speak--is only about 150 miles thick or a fiftieth of the earth's present diameter.”

”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 sh.e.l.l 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 readjustments 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.

”Of course the animals that lived on the earth in its flatter, more temperate stage now have to adapt themselves to life on high, cold elevations, or in dry, hot desert areas, or to migrate via the land-bridges to more favorable climates. Those unable to do this perished.

”For instance, take the age of reptile dominance, (the Mesozoic Era), which was in turn divided into four periods, those of dinosaurs, (the Tria.s.sic period, a rock from which I showed you, if you remember), the Jura.s.sic period, which gave rise to flying reptiles, from which our first birds were derived; the Comanchean period, which gave rise to flowering plants and the higher insects, and the Cretaceous period, when our most primitive mammal forms evolved.

”At first the earth was peopled with dinosaurs and flying dragons, and the seas by squid-like mollusks. In those days all the earth was level, swampy, tropic and overgrown with giant tree ferns and a primitive conifer.

”As the high mountain ranges arose and deserts were made, these forms gradually gave way to flowers and hardwood forests, peopled with insects and mammals. Only the most intelligent forms survived, and the struggle itself developed a higher degree of intelligence.”

”What in tarnation were _dinosaurs_?” asked Long Lester.

”Oh, haven't you ever seen pictures of them?” laughed Ace. ”Picture a giant lizard, perhaps 40 feet long----”

”Here, here,” protested the old man. ”I don't bite.”

”It is perfectly true,” said Norris soberly.

”Honest Injun!” vowed Ace. ”One of these fellows was a sort of cross between a crocodile and a kangaroo, what with his long hind legs that he could walk half erect on. There were some as small as eight or ten inches, too, and some so large that you wouldn't have come to his knee.

His big toe was as long as your arm.”

”And how do you know all that?” protested the old prospector feebly.

”By their bones,--fossils. Why, there have been fossil bones of a dinosaur found right in the Connecticut Valley! There was one found a hundred years ago in Oxford, England. We have heaps of fossils of them out West here. In fact, this part of the world used to be their stamping ground, though fossils of them have been found as far away as New Zealand.”

”Did they eat people?” gasped Lester.