But how could a river do this? It couldn't flow up one side of the mountain and down the other, could it? No, certainly not. What then? Wherever you find a river cutting through a mountain range you may be sure the river was there before the mountains rose, and that the mountains rose so slowly the river kept right on in its old channel and wore down the rock under that channel as fast as the mountains rose; while, on either side, they could rise as high as they wanted to for all the river cared! GROWING MOUNTAINS AND THE EARTHQUAKES But suppose, before I had explained how water-gaps are made I had told you I could show you a mountain growing. You wouldn't have believed it. Regions in which mountains are still rising, as on our Pacific Coast, are liable to earthquakes. The reason is that as mountains rise the rock layers of which they are made are strained dreadfully. Every once in a while they crack and the rocks on either side of this crack grind against each other. This makes the earth shake, much as the house shakes when a heavy table is pushed across a bare floor. If you want to see a job of river engineering that will make you catch your breath, look over into some of the river canyons and gorges of the West. [Illustration: THE GREAT CUMBERLAND WATER-GAP Here is the famous Cumberland Gap that the river cut through the mountains; so cutting a great figure in United States history, also, you remember. The picture shows the region as it looked in early days. ] A mile isn't much straight ahead, but a mile straight down and you on your stomach, with your eyes just over the edge--it's an _awful_ long way! Imagine yourself looking down a wall of rock like that, and the bottom of the abyss so far off that it looks blue--that's a canyon! AND YET THAT LITTLE RIVER DID IT ALL!