FRESH-WATER STREAMS. 647 



ing been swept off by erosion. Still farther to the east, beyond the 

 range of the view, another more elevated level is formed by Creta- 

 ceous strata with similar surface-features. All the immense amount 

 of erosion, indicated by these lofty, isolated remnants of strata that 

 once must have made part of a great elevated plateau, may be the 

 work of fresh waters alone. 



The facts brought forward show that, to produce a cluster or line 

 of crested mountain heights, with summits thousands of feet above 

 the plain around, it is only necessary that subterranean movements 

 should make a plateau of sufficient extent and elevation. Left ex- 

 posed to the rains, the carving will be all done in time. The Cats- 

 kills, as well as many mountain regions of the Rocky Mountain terri- 

 tories, and of other parts of the world, owe their features to the 

 eroding action of running waters. Mountains thus cut into shape by 

 water are sometimes called mountains of circumdenudation. 



Scotch valleys and mountains gave to Hutton the first right ideas 

 on this subject. 



When strata, of like durability, have considerable dip, erosion commonly results in 

 sloping surfaces, unless the rocks are so hard as to keep themselves in projecting ledges. 

 But if there is a stratum of easy removal alternating with others harder, as, for ex- 

 ample, a stratum of limestone among other kinds of metamorphic rocks, it is apt to 

 determine erosion, and make a valley, along its course, which will be the course of the 

 strike, and also to make, through consequent undermining, a high, precipitous, and 

 often rocky slope on the side toward which the rocks pitch, and a gradual slope on the 

 other; that is, if the dip is westward, the west side will commonly be the high steep side. 

 This principle is illustrated in many parts of the limestone region of western New Eng- 

 land and southeastern New York, where the dip is usually 45° to 70°. Some other points 

 with regard to the forms produced by erosion are illustrated on pages 651, 652. 



Many examples are on record of gorges, hundreds of feet deep, 

 cut out of the solid rock by two or three centuries only of work. 

 Lyell mentions the case of the Simeto, in Sicily, which had been 

 dammed up by an eruption of lavas in 1 603. In two and a half cen- 

 turies, it had excavated a channel fifty to several hundred feet deep, 

 and in some parts forty to fifty feet wide, although the rock is a hard 

 solid basalt. He also describes a gorge made in a deep bed of de- 

 composed rock, three and a half miles west of Milledgeville, Ga., 

 that was at first a mud-crack a yard deep in which the rains found a 

 chance to make a rill, but which, in twenty years, was 300 yards long, 

 20 to 180 feet wide and 55 feet deep ; and Liais describes a similar 

 gorge, of twice the length, in Brazil, made in forty years. High 

 floods on streams, with broad alluvial regions, often wear into the 

 earthy banks and commence new bends, or cut off old ones ; and, 

 sometimes, where broad plains border the sea, they open for them- 

 selves new channels for discharge. The great and turbulent Hoang- 



