When we consider the rate of sedimentation as a method for estimating geologic time, we take the total observed thickness of the geologic column, (estimated to be 70 miles) and divide it by the rate at which the sediments are now being laid down. But do we know this rate? By taking the average rate of sedimentation of nine large rivers[1] now in existence and assuming it to be the rate at which sediments were deposited in the past, an approximate conclusion can be arrived at. But it will be noticed from the figures of the rate of sedimentation of different rivers that they are widely variable, the highest being many times greater than the lowest. Can we then use an average of nine figures so out of proportion and yet expect a reliable quantitative value? Furthermore, the rate of deposition along the coast, near the mouth of a large and active river, is much higher than on a coastline where no rivers empty their sediments into the sea. The rate is also largely controlled by the character of the sediments deposited. Sandstone and shale are more rapidly deposited than limestone. Moreover, just as there is no knowledge of the duration of time of erosion between periods of sedimentation, there is also no record of the amount of detritus that has fallen off the edge of the continental shelf during widespread emergence of the continents. [1] -----------+-----------+-------------+-----------+------------------ River | Drainage | Total tons | Ratio of |Height in feet of | areas in | annually | sediment |column of sediment | square | | to water | with a base of | miles | | in weight | 1 square mile -----------+-----------+-------------+-----------+------------------ Potomac | 11,043 | 5,557,250 | 1: 3,575 | 4.0 Mississippi| 1,244,000 | 406,250,000 | 1: 1,500 | 241.4 Rio Grande | 30,000 | 3,830,000 | 1: 291 | 2.8 Uruguay | 150,000 | 14,782,500 | 1:10,000 | 10.6 Rhone | 34,800 | 36,000,000 | 1: 1,775 | 31.1 Po | 27,100 | 67,000,000 | 1: 900 | 59.0 Danube | 320,300 | 108,000,000 | 1: 2,800 | 93.2 Nile | 1,100,000 | 54,000,000 | 1: 2,050 | 38.8 Irrawaddy | 125,000 | 291,430,000 | 1: 1,610 | 209.0 -----------+-----------+-------------+-----------+------------------ Babb. Science, Vol. XXI, p. 343, 1893. We have also no record of the vast quantity of shallow water sediments that were stirred up by the penetration of storm waves, and carried to abyssal depths by the currents and tides.