18 



AQUEOUS AGENCIES. 



the emergence of this portion of the continent, became more rapid in 

 the latter portion of the Tertiary with the great elevation of the plateau, 



and has continued to the present time. 

 Thus, causes now in operation are identi- 

 fied with geological agencies. 



In the Appalachian chain gorges and 

 valleys of erosion are abundant, but the 

 evidences of present action are less obvi- 

 ous, and therefore we defer their treat- 

 ment to Part II, for we are now discussing 

 agencies still in operation. Among the 

 more remarkable narrow gorges in this 

 region, we may mention, in passing, the 

 Tallulah River gorge, several miles long 

 and nearly 1,000 feet deep, in Rabun 

 County, Georgia, and the gorge of the 

 French Broad in North Carolina. The 

 general effects of erosion will be more 

 fully treated under Mountain Sculpture 

 (page 255). 



Transportation and Distribution of 

 Sediments. 



The specific gravity of most rocks is 

 about 2 5. Immersed in water, they there- 

 fore lose nearly half their weight. This 

 fact greatly increases the transporting 

 power of water. The actual transport- 

 ing power of water is determined partly 

 by experiment and partly by reasoning on the general laws of force. 

 By experiment we determine the transporting power under a given set 

 of circumstances : by general reasoning we determine its law of varia- 

 tion, and apply the data given by experiment to every possible case. 



Experiments. — It has been found by experiment that a current, 

 moving at the rate of three inches per secoud, will take up and carry 

 along fine clay ; moving six inches per second, will carry fine sand ; 

 eight inches per second, coarse sand, the size of linseed ; twelve inches, 

 gravel ; twenty-four inches, pebbles ; three feet, angular stones of the 

 size of a hen's egg* It will be readily seen from the above that the 

 carrying -power increases much more rapidly than the velocity. For 

 instance, a current of twelve inches per second carries gravel, while a 

 current of three feet per second, only three times greater velocity, 



Fig. 11. 



-Section of the Virgen River 

 (after Gilbert). 



* Page's Geology, p. 28 — Rankinc. 



