en decidedly upward, while the trend of the ocean
floor has been downward. At that, the shell 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 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 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 oc