FRESH-WATER STREAMS. 651 



breadth of the stream, v its velocity, I the latitude, and w the angular 

 velocity of the earth about its axis (Cambridge Mathematical Tripos, 

 1875). With a stream 1,000 feet wide, the ratio for the impact on the 

 two sides at a depth of 10 feet, would be as 461 to 460. The effects 

 have beeu observed in many parts of the world where the deposits in- 

 tersected are earthy, and the pitch of the surface is very small and has 

 the direction of the stream. They are marked along the great rivers 

 of Siberia and Russia, on others in Southern France, on the streams 

 intersecting the low land of the Atlantic border of the United States 

 (Kerr), and on those of Southern Long Island (E. Lewis). 



5. 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 run up into lofty needles (or 

 aiguilles), 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 over all 

 sides of the peak, either from trickling waters or from frosts, and may gradually nar- 

 row it into the model aiyuille. But, when the rock has a cleavage-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 lofty domes of the Yosemite region 

 have been described as owing their forms to a cleavage or jointed structure in the gran- 

 ite, parallel with the surface. 



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 posi- 

 tion 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. 1079, 1080. 

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

 p. 6-i3. If the rock is firm, like most limestones, it may rise into lofty, few-angled sum- 

 Fig. 1081. Fig. 1082 



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. 



