754 J. BAERELL MEASUREMENTS OF GEOLOOIC TIME 



solved matter, one loot in 4,500 years. These figures have been given con- 

 siderable weight since the Mississippi has been taken as a representative 

 river.^ For the measurement of geologic time the mean rate of erosion 

 has generally been taken, however, as higher than this. Goodchild se- 

 lected one foot in 3,000 years as the best average. Sollas uses a figure 

 derived from Geikic, of one foot in 2,400 years. A more reliable mean 

 figure is based on solvent denudation. The mineral matter carried in 

 solution in river water, and also the qmintity of watei', can be accurately 

 determined at suitable intervals. By taking representative streams for 

 various climates and topographic reliefs, a mean result is obtained. Valu- 

 able figures for the area of the ITiuted States have been contributed es- 

 pecially by Dole and Stabler of the TJ. S. Geological Survey. On the basis 

 of these data F. W. Clarke finds that, taking the continents as a whole, 

 they are lowered by solvent denudation one foot in 30,000 years. This esti- 

 mate he regards as probably correct within 10 per cent.*^ From measure- 

 ments of the suspended matter collected in the analyzed samples it is found 

 for the United States that the removal of this insoluble material lowers the 

 mean continental surface one foot in 13,800 years. The rate for the com- 

 bined action is one inch in 760 years,'^ or one foot in 9,120 years. A num- 

 ber of writers who have used these data have failed to note, however, that 

 the materials transported along the bottom^ as sand or gravel, are not 

 reckoned into these results.^ In such a stream as the Mississippi near its 

 mouth this would iiivolve but a moderate correction, since the great bulk 

 of its material is there carried in suspension and solution. The silt, 

 however, is carried in greater quantity near the bottom. In applying the 

 method to the rate of erosion in the upper parts of river systems, or at 

 the mouths of streams which carry much sand to the sea, a larger correc- 

 tion for purely bottom transportation would be necessary. 



The rate varies greatly from season to season, and the volume of sus- 

 pended matter nf^ay yary 50 per cent from one year to another; but, con- 

 sidering the various factors, Dole and Stabler regard the figures for 

 denudation as gener9,Jly within 20 per cent of the true value. The rate is 

 highly contrasted, furthermore, in different regions. Measurement of dis- 

 solved and suspended matter at Yuma shows the Colorado basin, witb 

 an area of 230,000 square miles, to be lowered one foot in 5,300 years. 

 The western Gulf of Mexico drainage, with an area of 315,700 square 



^' A. Geikie : Text-book of Geology, .1903, pp. 589-591. 



" A preliminary stiuly of chemical rlenudatlon. Smithsonian Misc. Coll., vol. 50, no. 5, 

 1910. 



'' Dole and Stabler : Denudation. Water Supply Paper 2?A, U. S. Geol. Survey, 1909, 

 p. S3. 



'^ Dole : See methods of collecting samples. The quality of the surface waters in the 

 United States. Waler Supply Paper 2'Mi, U. S. Geol. Survey, 1909, p. 10. 



