448 THE POPULAR SCIENCE MONTHLY. 



they piled up in anotlier. The winds hlew the seeds of grasses 

 about, and willows and aspens crept up the mountain-sides. Then 

 came the squirrels, scattering the nuts of the pine. Other seeds 

 came, too, in other ways, till at last the barren hillside was no 

 longer barren. 



The brooks ran over the surface of the crust undisturbed by 

 the fires within, and were clear and cold as mountain brooks 

 should be ; but the rain and melted snow will never all remain on 

 the surface. Some of it falls into cracks or joints or porous jDlaces 

 in the rock, and from this come underground streams or springs. 

 But in this region a stream could not run long underground 

 without coming in contact with the old still-burning fires. When 

 a crust is formed over the lava, it cools very slowly. When the 

 crust is a rod or two deep, the lava within is almost as well pro- 

 tected as if it were at the center of the earth. 



Whenever the water came down into the fire, the hot rocks 

 would be furious with indignation, and tearing the water to atoms 

 they would throw it back to the surface as steam. Then the ex- 

 plosive force of the steam would in turn tear up the rocks, mak- 

 ing still larger the hole through which the water came. When 

 the rocks were very hot, a little water upon them would make a 

 terrible commotion like the shock of an earthquake. When much 

 water came down, it would hiss and boil high in the air, as it 

 tried to break the cushion of steam which came between it and 

 the lava. 



And all this went on in hundreds of places and maybe for 

 thousands of years. The hot rocks glowed and sweltered in the 

 ground, and the cold snow-water crept after them closer and closer, 

 while more and more vigorously the rocks resented the intrusion. 

 Sometimes the water would go down in a mass through a cleft, 

 when it would be hurled back bodily the very way it came. At 

 other times the water came down little by little, insinuating itself 

 into many places at once. Then the hot rocks threw it back in 

 many little honeycomb channels, and by the spreading of these 

 channels the rocks were at last crumbled to pieces. The hard 

 black lava or the glass-like obsidian were changed to white 

 kaolin as soft and powdery as chalk. And as the water fought 

 its way, gaining a little every year, steadily working between the 

 joints in the enemy's armor and as surely being thrown back with 

 violence if it penetrated too far, the animals and the plants 

 followed in the wake of the water, and took possession of the 

 territory as fast as it was won. 



At last the Pliocene times were over, for all times come to an 

 end. The one sure thing on the earth is the certainty of change. 

 With the change of time came on ihe earth's great winter. The 

 snow-drifts on the lava were piled up mountain-high. Snow is 



