654 



DYNAMICAL GEOLOGY. 



The eartby material and stones have all come from the rocks. And 

 among the agencies producing them, besides that of the wear and tear 

 from rivers, there have been also the wear and tear from the ocean's 

 waves and currents ; the wear and tear from moving ice or glaciers, 

 producing masses of all sizes, from that of a large house to the impal- 

 pable dust or clay that makes the outflowing waters milky for scores 

 of miles away ; the tearing action of freezing ; tearing by the roots of 

 growing trees (p. 607) ; the eroding and crumbling process of decom- 

 position (p. 704), — a much more efficient agent than all the above, 

 and largely preparatory for their action ; the tearing from varying tem- 

 perature ; and in volcanic regions the rending effects of volcanic forces 

 (p. 733). The rounded shapes of stones (often flattened, if the rock 

 whence they were derived was schistose or laminated) have come from 

 mutual abrasion while in the transporting water or ice ; but also from 

 surface decomposition, and the action of change of temperature. Stones 

 rarely come in rounded forms directly from the roqks, and when so it is 

 usually owing to decomposition along fractures (p. 87). 



Besides these materials, waters carry also matters in solution. 



2. Transporting Power of Water and its Effect on Decomposition. 



The transporting power of water increases rapidly with the velocity, 

 and partly in consequence of the fact that the weight of stones is a 

 third to a half less in water than out of it, — the loss being equal to 

 the weight of an equal volume of water. It is usually stated that in a 

 current exceeding \ ft. per second (about J m. per hour) fine mud or 

 silt will be moved, that is, scour will take place over such a bottom ; 

 exceeding \ ft. (^ m. per h.), clay; 1 ft. (§ m. per h.), sand; 2 ft. 

 (If m. per h.), gravel ; 4 ft. (2§ m. per h.), loose stones. According 

 to the experiments of M. Dubuat, a current of 4 miles per hour (or 

 6 ft. a second) will transport stones having a diameter of 2^ in. ; of 2 

 m. per h., 0.6 in. ; of -f m. per h., 0.064 in., or fine sand ; of J m. per 

 h., 0.016 in., or fine earth or clay. 



For material of like density, the mean diameter of the largest trans- 

 ported particles varies as the square of the velocity ; or, for any mate- 

 rial, the weight varies as the 6th power of the velocity. 



Experiments made in France in 1878 by M. Jacquet, engineer in chief, give greater 

 scouring force to a given current: a current of 1.09 per h. (1.64 ft. per second) transport- 

 ing particles 0.40 inch in diameter; of 2.18 m. per h., 1.60 inches; of 3.38 m. per h., 

 3.90 inches; of 4.36 m. per h., 6.70 inches. The rates are probably those that would 

 just move the particles along the bottom. 



In ordinary free transportation the stones or particles go over the bottom in a series of 

 long arches, though the finer material appears to be carried without sinking. The very 

 finest of clayey material sometimes remains suspended indefinitely, and because, accord- 

 ing to the experiments of Prof. W. T. Brewer, it is in a colloidal state, analogous to that 

 of gelatinous silica. This author found that such clouded waters became clear on freez- 

 ing. The addition of a saline solution greatly hastens the deposition of sediment. 



