1026 HISTORICAL GEOLOGY. 



facts as to the rate of downward increase of heat and the rate of loss of heat 

 into space; that if the earth had cooled less than 20,000,000 years since, 

 the internal heat would have been greater than now ; and if more than 

 400,000,000 years, there would have been no sensible increase of heat down- 

 ward ; and he suggested a probable length of 100,000,000 years. The bearing 

 of the facts as to tidal retardation, and his hypothesis with reference to the 

 origin and age of the sun's heat, gave him other arguments on the question y 

 but the conclusions are less well based than that from rate of cooling. 



Tait, arguing from the effect of tidal retardation on the figure of the 

 earth and the rate of cooling, concluded, that the time which can be allowed 

 to geologists is something less than 10,000,000 years ; and, in view of the 

 supposed origin of the sun's heat (by the falling together of masses of 

 matter), that the time the sun has been illuminating the earth is not more 

 than about 15,000,000 or 20,000,000 years ; and, again, that the supposed 

 concussion would have made heat enough to last the earth 100,000,000 years. 

 Croll has some speculations on this subject in Chapter xxi. of his Climate 

 and Time. 



Newcomb says : " If we reflect that a diminution of the solar heat by 

 less than one fourth its amount would probably mean an earth so cold that 

 all the water on its surface would freeze, while an increase of much more 

 than one half would probably boil all the water away, it must be admitted 

 that the balance of cause which would result in the sun radiating heat just 

 fast enough to preserve the earth in its present state has probably not 

 existed more than 10,000,000 years." 



The first of Kelvin's methods mentioned above has been adopted by 

 C. King (Am. Jour. Sc, 1893), with new data, derived from experiments by 

 C. Barus, with regard to the latent heat of fusion of the rock diabase, its 

 specific heat when melted and when solid, and its volume expansion between 

 the solid and melted state, besides other points bearing on the subject. The 

 conclusion reached is that the earth's age is probably 24,000,000 years. 



The safe conclusion from all the Geological and Physical facts is that 

 Time is long, very long ; long enough for the development of all the earth's 

 rocks, mountains, continents, and life. Geologists have no reason to feel 

 hampered in their speculations, while the extreme results of calculation are 

 10,000,000 ami 6,000,000,000 years. 



CLIMATAL DEVELOPMENT. 



A globe that has slowly cooled from fusion, and has had in the past, 

 as now, a sun that is losing heat like itself, must have been a globe also of 

 cooling climates. But its orbit has wide variations, and the sun, it is 

 supposed, its varying phases. Moreover, the present era is a time of mild 

 climate compared with that of the Glacial period which preceded it; and 

 hence the cooling of the climates has not been continuous and regular, but 

 one by oscillations, in which refrigeration was real, though often passing 

 through extremes in both directions. 



