WATERFALLS. 15 



The wonderful falls of the Yosemite Valley, of which there are six 

 in a radius of five miles, one of them 1,600 feet, three 600 to 700 feet, 

 and two over 400 feet high, seem to be an exception to the law given 

 above. Their perpendicularity seems to be the result of the compara- 

 tive recency of the evacuation of the valley by an ancient glacier, and 

 therefore the shortness of the time during which the rivers have been 

 falling, combined with the hardness of the granite rocks. The Yo- 

 semite gorge was not made by the present rivers during the present 

 epoch. 



Time necessary to excavate Niagara Gorge. — All attempts to esti- 

 mate accurately the time consumed in excavating Niagara gorge must 

 be unreliable, since we do not yet know the circumstances which con- 

 trolled the rate of recession at different stages of its progress. Among 

 these circumstances, the most important are the volume of water, and 

 especially the hardness of the rocks, and the manner in which hard and 

 soft are superposed. The present position of the falls is apparently 

 favorable for rapid recession. Mr. Lyell thinks, from personal observa- 

 tion, that the average rate could not have been more than one foot per 

 annum, and probably much less. At this rate it would require about 

 36,000 years. More recent estimates make the probable rate three feet 

 a year, and the time, therefore, 12,000 years. But whether we adopt 

 the one or the other estimate, this time must not be confounded with 

 the age of the earth. The work of excavating the Niagara chasm be- 

 longs to the present epoch, and the time is absolutely insignificant in 

 comparison with the inconceivable ages of which we will speak in the 

 subsequent parts of this work. The Falls of St. Anthony recedes about 

 five feet per annum, and has made its gorge in about 8,000 years 

 (Winchell). 



Ravines, Gorges, Canons. — We have already seen (page 12) that 

 ravines, gorges, etc., are everywhere produced in mountain-regions by 

 the regular operation of erosive agents. Nowhere are examples more 

 abundant or more conspicuous than in our own country, and especially 

 in the Western portion. On the Pacific slope, the most remarkable are 

 the gorges of the Fraser and of the Columbia Rivers, fifty miles long 

 and several thousand feet deep ; those of the North and South Forks 

 of the American River, 2,000 to 3,000 feet deep in solid slate ; the canon 

 of the Tuolumne River, with its Hetclilietcliy Valley ; the cafion of the 

 Merced, with its Yosemite Valley, with nearly vertical granite cliffs, 

 3,000 to nearly 5,000 feet high ; and, deepest of all, the grand carton 

 of King's River, 3,000 to 7,000 feet deep, in hard granite. 



Some of these great canons have been forming ever since the forma- 

 tion of the Sierra Range — i. e., since the Jurassic period. It is possible, 

 also, that in some of them the erosive agents have been assisted by 

 antecedent igneous agencies, producing fissures, which have been en- 



