ON BIOLOGY. 41 



positing over half an inch, our estimate should read only 

 about one-eighth of an inch, at which rate it would require 

 about ten thousand years to make a layer one foot thick. 

 In any coal basin with an aggregate thickness of 100 feet 

 its formation must have required one million 

 years to accomplish. But it is not uncommon 

 to find 150 feet to be the average thickness in 

 some coal measures, and a proportionately 

 longer time must have been required. This method of 

 computation takes into consideration the rate at which a 

 vigorous vegetation produces organic matter, but we may 

 also arrive at a solution by estimating the rate at which 

 the river deposits its sediments over the area in which 

 the coal is forming. 



I will conclude this part of my subject with an ex- 

 ample of this nature, presented us by the authority 

 quoted in several instances above, and we are told that 

 our indebtedness is to Sir Charles Lyell for the "estimate 

 of the time necessary to accumulate the Nova Scotia coal 

 measures. This coal-field is selected because the evi- 

 dences of river sediments are very clear throughout. 

 The area of this coal-basin is 18,000 square miles; but the 

 identity in character of portions now widely separated 

 by seas e. g., on Prince Edward's Island, Cape Breton, 

 Magdalen Island, etc. plainly shows that ail these are 

 parts of one original field, which could not have been less 

 than 30,000 square miles. 



"At Pictou, the thickness is nearly 13,000 feet, and we 

 certainly shall not err on the side of excess, therefore, if 

 we take the average thickness over the whole area of 

 7,500 feet. This would give the cubic contents of the 

 original delta deposit as about 51,000 cubic miles. Now, 

 the Mississippi River, according to Humphrey and Ab- 

 bott, carries to iis delta annually sediment enough ta 

 cover a square mile 268 feet deep, or nearly exactly one- 

 twentieth of a cubic mile. Therefore, to accumulate the 

 mass of sediment mentioned above would take the Mis- 

 sissippi about 1,000,000 years." 



And, mark you, in the geological series of the earth's 

 crust the Carboniferous period is not more than one- 

 thirtieth of her recorded history. Then we must believe 

 that that history covers a period of 30,000,000 of years. 

 But Mr. Wallace, by a most careful estimate made from 

 the premises of the general erosion of the land area of 

 the earth, makes its recorded history but 28,000,000 of 

 years. Here is a difference of 2,000,000 of years, as well 

 as a difference of opinion, but it can, nevertheless, hardly 



