Erewhon by Samuel Butler

My next business was to descend upon the river. I had lost sight of the pass which I had seen from the saddle, but had made such notes of it that I could not fail to find it. I was bruised and stiff, and my boots had begun to give, for I had been going on rough ground for more than three weeks; but, as the day wore on, and I found myself descending without serious difficulty, I became easier. In a couple of hours I got among pine forests where there was little undergrowth, and descended quickly till I reached the edge of another precipice, which gave me a great deal of trouble, though I eventually managed to avoid it. By about three or four o'clock I found myself on the riverbed.

From calculations which I made as to the height of the valley on the other side the saddle over which I had come, I concluded that the saddle itself could not be less than nine thousand feet high; and I should think that the riverbed, on to which I now descended, was three thousand feet above the sea-level. The water had a terrific current, with a fall of not less than forty to fifty feet per mile. It was certainly the river next to the northward of that which flowed past my master's run, and would have to go through an impassable gorge (as is commonly the case with the rivers of that country) before it came upon known parts. It was reckoned to be nearly two thousand feet above the sea-level where it came out of the gorge on to the plains.