40 LECTURES 



imagination how long it took the Colorado River to find 

 its present bed, a mile below the surface of the earth, and 

 for a distance of 300 miles in length. 



Deltas of rivers and their formations furnish us with 

 more of Nature's operations from which we may make 

 toleratly correct estimates of the time it takes for such 

 feats to be performed. The delta of the Mississippi 

 could not have taken less than 50,000 years and probably 

 took a very much greater time to form. Here we have 

 for data the cubical contents of the delta; the annual 

 mud-discharge of the river; and the computation of the 

 extent of the submarine portion of the delta, and some 

 few minor factors. 



Again, the history of the coal beds and the accumula- 

 tion of coal oifer us with another series of facts from 

 which it is possible to gain some idea from another point 

 of view of the enormous lapse of time it has taken Nature 

 to achieve some of her works. 



It is now perfectly demonstrable that coal was accumu- 

 lated during the Carboniferous period, and the accumula- 

 tion and formation of it took place at the mouths of 

 certain great rivers, which at that time discharged 

 themselves into the ocean. There, in such places, existed 

 vast peat swamps, overgrown by the peculiar vegetation 

 of that period, which at all times were subject to floods 

 from the river on the one hand, and inundations from 

 high oceanic tides on the other. A recent coal bed, of 

 identically the same nature, is now in the process of 

 formation under our very eyes in the Mississippi delta, 

 and careful study is alone required to decide the rate at 

 which coal is deposited therein. Oiher forces are and 

 were also at work both during recent time and during the 

 geologic period or subperiodsof the Carboniferous system. 

 These have been scientifically considered by many com- 

 petent geologists, and their operations taken duly into 

 consideration, but it is not necessary for us to dwell upon 

 them here, and to many of you they are no doubt already 

 familiar. It has been ascertained, for example, that a 

 vigorous vegetation yields by death and decay and 

 growth about 100 tons of dried organic matter per cen- 

 tury to the acre. But such an amount of vegetable mat- 

 tjr pressed to the specific gravity of coal would make a 

 layer only a little over half an inch in thickness, when 

 spread over the area just mentioned. It must not be for- 

 gotten, however, that certain chemical losses are experi- 

 enced during such a process, and upon giving these due 

 weight the result has been arrived at that, instead of de- 



