680 DYNAMICAL GEOLOGY. 



Europe and the other continents furnish others no less remarkable, 

 and to an indefinite extent. 



2. Topographical effects of erosion. — The topographical effects 

 of erosion depend on several conditions, — as (1) the durability of 

 the rocks, (2) their structure, and (3) their stratification. 



1. Durability of the rocks. — Granite is well known to runup into lofty needles 

 (or aiguilles), as in the Alps and, still better, the Organ Mountains of Brazil and 

 some peaks in the Shasta Mountains of California. But there are varieties 

 crumbling easily on exposure, and these occur only in broad, massive eleva- 

 tions. The hard argillite (roofing-slate) often forms bold, craggy heights, while 

 soft argillaceous shales make only tame hills and undulating plains. 



The refractory quartzites 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 over all sides of the peak, either from trickling waters or frosts, and may 

 gradually narrow it into the model aiguille. But when the rock has a cleav- 

 age-structure like the schists and slates, its heights are rough and angular, 

 and its aiguilles, 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 

 battlements when exposed in cliffs (fig. 88, p. 100). The architectural effect of 

 the columnar cleavages of trap or basalt is shown in fig. 115, p. 118. 



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 through- 

 out, the elevations have generally table summits, with vertical rocky brows 

 facing the lower lands. The river-valleys are profound, and often inaccessi- 

 ble for long distances, owing to the boldness of the precipices. Some varieties 

 of these valleys are shown in figs. 940, 941. Other topographical effects are 

 described in the remarks on the erosion of valleys, p. 635. If the rock is 

 firm, like most limestones, it may rise into lofty, few-angled summits, espe- 

 cially 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 atop 

 of the harder rock, and the declivities are usually banded with projecting 

 shelves and intervening slopes. Figs. 954, 955 represent the common cha- 



Fig. 954. Fig. 955. 



raeter of such hills. A number are shown in fig. 940 ; in the Colorado region 

 they have been called Mesas, from the Spanish for table* 



* For figures 954-965 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. 



