GENERAL OBSERVATIONS. 1025 



drainage area in about 6000 years. The rate usually taken is one foot in 

 3000 to 7000 years. The rate 1 to 3000 was deduced by S. Haughton (1880) 

 from the rate of sedimentation of the following rivers : the Mississippi, 6000 

 years ; Ganges, 2358 ; Hoang Ho, 1464 ; Yang-tse-Kiang, 2700 ; Khone, 1526 ; 

 Danube, 6846 ; Po, 729 years ; the mean of which is 3090 years. He adds 

 that since the sea bottoms are to the land surfaces as 145 to 52, the rate at 

 which the sea bottoms are becoming silted up, that is to say the present rate 

 of formation of strata, is one foot in 8616 years. Thence, supposing the rate 

 the same as now for all past time from the Archaean onward, the whole 

 duration of geological time is 200,000,000 years. But Haughton included in 

 the thickness of the terranes, 60,750 feet of Archaean, or over one third of 

 his total (177,200 feet). Deducting for the Archaean, the length of the rest 

 of geological time would be about 130,000,000 years. 



Mellard Reade makes the time since the Archaean, on the same kind of 

 basis (taking the mean area of denudation as one third the entire land area, 

 and the rate of denudation one foot in 3000 years), 95,000,000 years (1893). 



C. D. Walcott deduces, for the elapsed time, 70,000,000 years (1893) ; 

 H. H. Hutchinson, 600,000,000 years (1892) ; M'Gee, including in his basis 

 the rate of denudation at Niagara, and giving credit to the extreme estimates 

 of thickness of the early Paleozoic formations, 6,000,000,000 years (1893). 



All these estimates proceed on the solid basis of existing facts. Yet in 

 deriving them the extreme difference between the existing earth and that of 

 the geological past was not taken into account. Going back in geological 

 time, the rock-making portion becomes more and more widely marine, and 

 rivers have correspondingly diminished size and drainage areas. But, at 

 the same time, climates become warmer and precipitation therefore increas- 

 ingly abundant ; and through Paleozoic to earlier time the eroding carbonic 

 acid and oxygen of the atmosphere are increased in amount, and corro- 

 sion thereby was proportionately greater. Even as late as the Middle 

 Cretaceous, the western half of North America was an open sea with its 

 large and small islands. In the Paleozoic, and still more in the Archaean, 

 the whole continent was in a like condition ; and the Continental seas had 

 only little streams and drainage areas to supply sediment for the thick 

 formations, so that the sea did much more than half the work in its slow 

 way. Further, changing climates have occasioned changing rates of erosion 

 and sedimentary deposition ; and have made, over large continental areas, 

 times of great precipitation to alternate with times of prevailing drought, 

 and times of full lakes and of large hard-working rivers, with times of 

 dwindled or feeble waters. In addition, the deposits of one period have 

 often been largely denuded to make those of the following; and the chief 

 sources of all sediments are Archaean. Attempts therefore to find, in the 

 results of aqueous action, a definite measure of any part of the geological 

 past necessarily lead to very doubtful results. 



Length of geological time on evidence from terrestrial physics. — Kelvin 

 pointed out in 1862 that a limit to the earth's age is fixed by the known 



DANA'S JIANUAT- — Go 



