xUv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



miles, would be of such a thickness, that to accumulate it to a depth 

 of one-tenth of a mile, a period of 67,000 years must have elapsed ; 

 and supposing the alluvial matter in the plain above the delta to have 

 only one-half the above-estimated depth, and to be only of the same 

 area as the delta (although it is somewhat greater), and that the 

 solid contents of the year were spread over the united area, a period 

 of 100,500 years would be required for the formation of the whole 

 plain. The proportion of the thickness that would be derived from 

 the coarser unsuspended materials and the drift wood, vast in amount 

 as the latter is every year, Mr. Lyell holds to be more than compen- 

 sated in the calculation, by the quantity of suspended matter which 

 would not fall down before the river-water Avas carried far out to sea. 

 The depth of the alluvial soil above the head of the delta is, in the 

 absence of borings, estimated from this, that the river is continually 

 shifting its course in the great plain, cutting frequently to the depth 

 of 100, and sometimes to the depth of 250 feet. 



This calculation can only be considered as a first attempt to give 

 an approximate numerical value to a part of one of the periods in 

 geological chronology, and that period the most modern in the series. 

 Throughout the whole range of geological changes, from the lowest 

 sedimentary stratum to that of deposits within the historical time, 

 an attentive study of the phsenomena of each impresses us with a 

 conviction, almost amounting to a demonstrated truth, that a vast 

 lapse of time is indicated by each great successive change — ^periods 

 however to which we can assign no definite amount, from the absence 

 of the necessary data by which we can obtain an unit. But if a 

 geological change be in progress in our own time, such as the deposit 

 of an alluvial soil by a river, and we are able to estimate the amount, 

 say one foot in thickness, between two fixed periods a and b, b being 

 the earlier period, if the total deposit be 50 feet thick, and be of an 

 uniform composition and character throughout, it is fair to infer that 

 each of the 49 feet below b must have required the same time for 

 its deposition as that between a and b ; unless it can be shown that 

 there are circumstances which would cause an acceleration or a re- 

 tardation of the process. If, as in Egypt, there were in the valley 

 of the Mississippi monuments of human art of remote antiquity, the 

 age of which was pretty accurately known, round which the alluvial 

 matter was accumulated, the monuments resting on a soil of the same 

 nature, we should have a better measure, a standard scale of some 

 accuracy, to begin with ; and if we had sections or borings at various 

 points in the valley, by which we could ascertain the depth and na- 

 ture of the alluvial deposit, and whether the bottom of the valley 

 was level or sloped gradually from the Gulf of Mexico to the head of 

 the plain, we should be able to form a tolerably correct estimate. 

 As it is, the above calculation can only be considered as a reasonable 

 deduction from the limited data we have yet obtained ; and it will 

 no doubt serve as a stimulus to future observers to collect materials 

 for the working out of a problem so interesting and important, not 

 only in the valley of the Mississippi, but in other localities favour- 

 able for such inquiries. 



