. Elements of geology : a text-book for colleges and for the general reader. Geology. 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. Amo
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 twelv
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