TOPOGRAPHICAL EFFECTS OF EROSION. 645 



1. Durability of the Rocks. — Granite is well known to run up into lofty needles (or 

 aiyuilles), as in the Alps and, still better, the Organ Mountains of Brazil, and some 

 peaks in the Castle Rock range, a few miles southwest of Mount Shasta, California. 

 But there are varieties crumbling easily on exposure; and these occur only in broad, 

 massive elevations. The hard argillyte (roofing-slate) often forms bold, craggy heights, 

 while soft argillaceous shales make only tame hills and undulating plains. 



The refractory quartzytes and grits, which make little or no soil, stand up in rude 

 piles and massy brows of nearly bare rock. 



2. Structure. — When there are no planes of structure, as in true granite, the rock 

 may rise into lofty peaks, with rounded surfaces. Slow denudation goes on overall sides 

 of the peak, either from trickling waters or from frosts, and may gradually narrow it into 

 the model aiguille. But, when the rock has a cleavage-structure, like the schists and 

 slates, its heights are rough and angular, and its aiyuilles, if any are formed, are more 

 apt to be pyramidal than conical. 



The joints in slates or sandstones often lead to forms resembling walls and battle- 

 ments, when exposed in cliffs (Fig. 88, p. 88). The architectural effect of the columnar 

 cleavages of trap or basalt is shown in Fig. 115, p. 108. 



•3. Stratification. — The results with stratified rocks differ according to (1) the position 

 of the strata, and (2) their nature. 



If the strata are horizontal, or nearly so, and hard and similarly so throughout, the 

 elevations have generally table summits, with vertical, rocky brows facing the lower 

 lands. The river-valleys are profound, and often inaccessible for long distances, owing 

 to the boldness of the precipices. The flooded waters of the valley wear the rocks at 

 the base of the precipice, and so undermine it, and make avalanches of rock which keep 

 the front nearly vertical. Some varieties of these valleys are shown in Figs. 1078, 1079. 

 Other topographical effects are described, in the remarks on the erosion of valleys, p. 

 640. If the rock is firm, like most limestones, it may rise into lofty, few-angled sum- 

 mits, especially when erosion has been preceded by fractures; as in the Alpine heights 

 of the Wetterhorn and its associates, near Grindelwald, in the Bernese Oberland. 



If horizontal, or nearly so, but of unequal hardness, the softer strata are easily worn 

 away, undermining the harder strata; the table-lands have a top of the harder rock, 

 and the declivities are usually banded with projecting shelves and intervening slopes. 

 Figs. 1080, 1081 represent the common character of such hills. A number are shown 



Fig. 1080. Fiff. 1081. 



in Fig. 1079; in the Colorado region, they have been called Mesas, from the Spanish for 

 tabled In some parts of the Rocky Mountain slopes, the thick gravel deposits are covered 

 with streams of lava of great thickness ; and table mountains are common is such regions. 



Elevations thus left prominent, after denudation around, have been called hills, or 

 mountains, of circumd emulation. Figs 1082, 108:5 are other examples. 



When the beds are inclined between 5° and 30°, and are alike in hardness, there is a 

 tendency to make hills with a long back slope and bold front; but, with a much larger 

 dip, the rocks, if hard, often outcrop in naked ledges. 



When the dipping strata are of unequal hardness, and lie in folds, there is a wide 

 diversity in the results on the features of elevations. 



Figs. 1082, 1083 represent the effects from the erosion of a. synclinal elevation con- 

 sisting of alternations of hard and soft strata. The protection of the softer beds by the 

 harder is well shown. This is still further exhibited in Fijrs. 1084-1087. 



1 For Figs. 1080-1091, and the views they illustrate, the author is indebted to the 

 volume on Coal and its Topography, by Lesley. In a long chapter on "Topography 

 as a science," this author has given the results of extensive personal observation. 



