WATER AS A MECHANICAL AGENT. 185 



6. Waterfalls. — The facts reviewed show that waterfalls are often a conse- 

 quence of the alternation of hard and soft strata in the course of flowing 

 waters. The hard strata resist downward wear; the soft yield easily. 

 Down the waters go, working with new force from the fall ; hence they un- 

 dermine the hard bed and thereby steepen the descent often to a vertical or 

 even an overhanging front. The columns made by drops (page 178) partly 

 illustrate the principle. 



The waterfalls about the head waters of rivers in the mountains have a 

 different origin ; for the lofty precipices may be cut out of a single block of 

 rock, as in the case of the central portion of Tahiti. These precipitous walls 

 are a consequence of the prolonged erosion of a region until a larger part of 

 the vertical descent of the stream is made at or near its head. 



Waterfalls far down the courses of rivers, like that of Niagara, are looked 

 upon as evidence of the recency of that part of the channel which contains 

 the fall (Powell). But those about the source in the mountains may be, 

 on the contrary, a final result after a long era of erosion ; not the ultimate 

 result, for the last end of the work would be the degradation and removal 

 of the crested heights. 



7. Features of mountains ; Forms made by water-sculpture. — Elevations of 

 all kinds have derived their existing features largely through water-sculp- 

 ture. Tahiti was originally a lofty mountain, probably twice its present 

 height, with low, nearly even, downward slopes in all directions, and only 

 small unevennesses from the piling here and there of lavas throiigh localized 

 eruptions. It now is a mountain of peaks, crested ridges with lofty preci- 

 pices, and vertical lines in all the features. But water has no need of a 

 mountain mass to make the grandest of so-called mountains. It will work 

 an elevated plateau, horizontal in surface, into mountain forms, and so make 

 mountains without any upturning or uplifting except that of the plateau. 



The chief part of the features produced come from the alternation of 

 hard and soft strata among the stratified rocks ; and these are greatly varied 

 by the positions of the strata. The elements of this system of architecture 

 are well illustrated in the figures on page 186 by Lesley, taken from his 

 work on Coal and its Topography (1856), in which the author' has given 

 the results of extensive personal observation in the Appalachian region. The 

 harder strata may be hard sandstone or limestone, and the softer, shale or 

 crumbling sandstone. The first figure (165) illustrates the origin of a 

 "table mountain" or "mesa" (Spanish for table), a hard layer making the 

 top, and, by resisting wear, protecting the softer beds directly below it. The 

 other figures illustrate other effects, under the same principle, in rocks having 

 various positions. Figs. 166 to 172 are synclines, and 173 to 176, anticlines, 

 of different forms, in three of which a valley has the place of the upward 

 bend — a common fact in the Appalachian Mountains. 



Monument Park in Colorado is a region of Tertiary sandstone carved into 

 monumental forms by denuding processes, the winds having given finishing 

 touches. As the view shows, the thin, harder layers in the sandstone make 

 the caps and moldings of the monuments. 



