130 Mr. J. Croll on the Eccentricity of the Earth's Orbit. 



earth has of getting quit of the heat received from the sun, and of 

 thus retaining itself at a much lower temperature than it would 

 otherwise be. It is in the equatorial regions that the earth loses 

 as well as gains the greater part of its heat. So of all places it is 

 here that we ought to place the substance best adapted for pre- 

 venting the dissipation of the earth's heat into space, if we wish to 

 raise the general temperature of the earth. Water, of all sub- 

 stances in nature, seems to possess this quality to the greatest 

 extent; and, besides, it is a fluid, and therefore adapted by means 

 of currents to carry the heat which it receives from the sun to 

 every corner of the globe. 



In assuming those three periods of great excentricity between 

 1,000,000 and 700,000 years ago to be those of the true unstra- 

 tified boulder-clay (Lower Till) , there is one slight difficulty that 

 meets us. It seems to place the glacial epoch too far back. Can 

 it be really 700,000 years since the close of the period of the 

 boulder-clay ? 



When we look at a gorge several thousand feet in depth that 

 has been cut out of the solid rock since the period of the boulder- 

 clay by a small streamlet, and reflect that this streamlet will run 

 for centuries without producing any perceptible effect on its 

 rocky bed, our first impression would be that 700,000 years is but 

 a short period for such a feeble agent to perform such an enor- 

 mous amount of work. But we are deceived. Time, as repre- 

 sented by geological phenomena, is deeply impressive ; and when 

 we attempt to express it in figures we are apt to be misled ; for 

 we can form but a very inadequate conception of immense dura- 

 tion represented in numbers. If a stream were to deepen its 

 channel only one-tenth of an inch in a year, it would in 700,000 

 years cut a gorge nearly 6000 feet deep. It would deepen its 

 channel nearly 600 feet were it to scoop out only an inch in a 

 century. 



The quantity of sediment discharged into the sea annually by 

 the Mississippi river is 28,188,083,892 cubic feet. The area of 

 drainage is 39,029,760,000,000 square feet*. Consequently 1 

 foot is being removed off the face of the country every 1388 years 

 and carried into the sea. If the rate of denudation be as great 

 in this country as in America, then 500 feet must have been 

 removed off the face of the country and carried by our rivers 

 into the sea since the period of the boulder-clay, if we place that 

 period 700,000 years back. Humboldt thinks that the mean 

 elevation of all the land is less than 1000 feet. According to 

 the rate at which the rivers are carrying the land into the sea, 

 if there be no more elevations of the land, our continents will 



* See " Report on the Sediment of the Mississippi River," Proceedings 

 of the American Association for the Advancement of Science, 1849. 



