226 SEE— FURTHER RESEARCHES ON [April 24, 



range varies from about 60 to 300 or more miles, but, as compared with 

 other mountains, the Andes are for the most part narrow relatively to their 

 height. Where their special features are most characteristically developed, 

 they consist of a massive embankment-like foundation, rising with a rapid 

 slope from the low country on either side, and having its margins sur- 

 mounted by lofty ridges of ragged or dome-like summits. These Cor- 

 dilleras, as they are usually termed, flank longitudinal valleys, or plain- 

 like depressions which form the highest levels of the central portion of 

 the gigantic embankment, and which vary in width from twenty to sixty 

 miles. At intervals the longitudinal depression is broken up, either by ridges 

 connecting the Cordilleras, or by lofty plateau-like uplands. In several 

 cases these transverse ridges and belts of high ground form the main 

 watershed of the country. They are rarely cut across by the river systems, 

 whereas both the marginal Cordilleras are intersected at numerous points, 

 and more especially by the rivers draining the eastern slope of the country. 

 In no case do these eastern rivers originate to the west of the western 

 Cordilleras. A few of the central valleys, or plain-like depressions, have 

 no connection either with the western or eastern river system. Roughly 

 speaking the height of the central plains or valleys is from 6000 to 11,000 

 feet above the sea; of the passes and knots, from 10,000 to 15,000 feet; 

 and of the highest peaks, from 18,000 to 23,290 feet — the last being the 

 altitude of Aconcagua in Chili, which is generally considered to be the 

 highest peak in America. Judging from these estimates, we may regard 

 the bulk of the Andes as somewhere about that of a mass 4400 miles long, 

 100 miles wide, and 13,000 feet high, which is equivalent to 5,349,801,600,- 

 000,000 cubic feet. On this basis we find that the Mississippi would carry 

 down an equivalent mass of matter in 785,000 years. The rate of denuda- 

 tion in certain river basins varies from one foot in 700 years to one foot 

 in 12,000 years. Assuming that similar rates would apply to the Andes, 

 they would be denuded away in from 9 to 156 million years. In all proba- 

 bility, much less than 9 million would suflice. On the other hand the 

 Andes would be swept away in 135,000 years, supposing the denuding powers 

 of the globe were concentrated on them alone. From the above data, and 

 assuming the average specific gravity of the matter forming the Andes to 

 be 2.5, the weight of the portion above the sea may be estimated at 368,- 

 951,834,482,750 tons, giving an average of about 1,000 tons on each square 

 foot at the level of the sea. Under Aconcagua the pressure would be about 

 1,780 tons per foot at the same level, provided, of course, it were not, as 

 it no doubt is, more or less modified by lateral pressure. These figures 

 afford some, though at best a vague, conception of the mighty grandeur of 

 this range of mountains, and of the scope there is for the exertion of 

 enormous pressure. How vast then, must be those forces which have 

 counteracted such pressures, and upheaved the ocean-spread sediments of 

 the continents, until the Andes, that 



' giant of the Western Star, 

 Looks from his throne of clouds 

 O'er half the world ! ' 



