“I swan!” was all the old prospector could say.
“Yes, the first mammals developed from a reptile known as the cynodont. Many of these reptiles had long legs and could travel with the body well off the ground. Birds originated from the same reptilian stock as did the dinosaurs. First their hind-legs grew long so that they could run on them,—and you will notice at the Museum how the legs of a dinosaur are joined to the body exactly like a bird's,—then their scales gradually evolved into feathers.
“There is a lot more to it than I can tell you now, but after various ups and downs, dinosaurs became extinct and Nature tried out several kinds of warm-blooded, furry mammals, some of them herbivorous and built for speed to run away from their enemies, some of them swamp-dwelling monsters with heavy legs and small brains, who, slow of movement, relied on horns and other armor and sharp teeth for their defense.
“But there is no end to this subject. I only mean to make the point that it was geological changes that drove the fish to land, and the land animal to higher forms, till finally other geological changes drove man’s ancestors down out of the trees.” The boys, no less than the old prospector, testifying their interest in the last named operation, he continued.
“When the Alps, the Andes and the Himalayas arose, man’s ancestors still lived in trees. But high mountains hold a large part of the moisture of the atmosphere in the form of snow and ice, and at the same time the decreased oceanic areas offer less surface for evaporation. Not only does that mean a drier climate, but the sun’s rays pass more freely through dry air, and the days are hotter, and the heat passing freely back through the same dry air at night, the nights are colder. Seasons are more extreme, and ice accumulates on the mountain tops and around the polar region, precursor of a glacier period. The aridity decreases the amount of forest, and the manlike tree dweller had to descend to the ground to get his living. That necessitated the development of his hind legs for speed, and that speed necessitated his assuming a wholly erect posture. That in turn freed his hands, and he, or the man descended from him, could defend himself by throwing stones at the huge beasts who then peopled the earth. The cold winters necessitated the use of the skins of beasts for clothing, and so on through the list. It was geological necessity that drove man into his higher development.
“Changes of climate and environment, however, are stimulating, even to-day. Statistics show that stormy weather actually increases people’s energy.”
The next day they passed a long crack in a rock slope, which Norris felt sure had been made by an earthquake, perhaps as recent as that of 1906, to judge from the cleanness and newness of it. The crack was no more than a foot or two in width, but in places eight feet deep, they estimated, and along the Western side of it stood a fault scarp, in this case a wall of granite bowlders of various sizes up to four or five feet in height.
“This,” pronounced the geology man, “is evidently a region overlying subterranean volcanoes, which might even yet build the range higher. I’ll bet that kind of mountain building may still be going on around here.”
Again and again Norris, or even Ace, had been able to point out, in the record of the rocks, the evidences of the two glacier periods that had helped shape the Sierra Nevada, the earlier one much larger, and enduring longer, as shown by the moraines (or deposits) left behind. The lower end of a canyon would be no wider than the stream that incised it, but the upper portion would have been smoothed into grassy parks or lakelets on each tread of a giant stairway to the summit of the range.
Rounded waterworn pebbles and cobblestones among a mass of angular bowlders, left behind by glacier streams, together with an occasional striated pebble, were “sermons in stones” to the geologist.