C. Lloyd Morgan — Geological Time. 201 



dragged along the bottom. In the above calculations, therefore, of 

 the rate at which the general surface of large areas is being lowered, 

 we may fairly consider that the length of time mentioned as that 

 in which the several rivers would remove one foot of solid matter 

 may be reduced by, say, one-fifth, in some cases more, and in some, 

 perhaps, less. 



Let us now see the application of these facts. The area of the 

 Mississippi basin is 1,147,000 square miles, that of the delta of 

 the same river 12,300 square miles. Let us assume that the area 

 over which deposit is now taking place is equal in size to the 

 present delta, and let us assume that only three-fourths of the 

 material swept off the Mississippi basin is thus deposited, the other 

 one-fourth being carried far into the Gulf of Mexico. Then -roVo 

 of a foot is swept from the basin and deposited over, say, 12,000 

 square miles. This will form a deposit rather more than one- 

 seventh of an inch thick. One-seventh of an inch per annum may 

 therefore be taken to be the rate of deposit in the delta of the 

 Mississippi. According to the estimates of Colonel Strachey, based 

 on the observations of Mr. Everest, the rate of deposit is more rapid 

 in the delta of the Ganges and Brahmapootra, amounting to more 

 than one-fifth of an inch per annum, and this over an area of 

 65,000 square miles. The Po drains an area of about 30,000 

 square miles, and lowers that area yi-g- of a foot every year. If 

 we take the area of deposit at the mouth of the river at 1000 square 

 miles, the rate of that deposit will be nearly half an inch per 

 annum. To take one more case, excavations were carried on by 

 Mr. L. Horner, in 1850, in the great plain of Memphis, to ascertain 

 what thickness of Nile mud had been deposited there since the 

 erection of the colossal statue of Rameses II. It was found that 

 nine feet four inches of silt had accumulated since the foundations 

 of the statue had been laid some 3211 years previously. This 

 is at the rate of -aVth of an inch per annum. Thirty-two feet below 

 the base of the pedestal the Nile mud was found to rest on desert 

 sand, and close to the line of junction a fragment of burnt brick was 

 discovered, which, if the rate of deposit was the same before the 

 erection of the statue as since its foundations were laid, was 13,496 

 years old. The rate of deposit at Elephantine, near the First 

 Cataract, was estimated by Sir J. G. Wilkinson at nearly double 

 that near Memphis, or about one-sixteenth of an inch per annum, 

 while the high Nile has been known to cause a deposit one inch in 

 depth for ten feet of water. 



Taking all these facts into consideration, we shall scarcely err 

 in excess if we estimate the average rate of delta deposition at 

 one-tenth of an inch per annum. In taking the present rate of 

 deposition as our standard, we are not likely to err in excess : for 

 though in past ages the average amount of rainfall in our hemisphere 

 may have been at some times less than the present amount, yet at 

 other times, during periods of higher excentricitj' of the earth's orbit, 

 for example, the amount was, probably, considerably greater. 



With regard to estuarine and lake deposits, we have not many 



