GEOLOGY. 265 



ON THE CAUSES OF THE CHANGE OF CLIMATE AT DIFFERENT 



GEOLOGICAL PERIODS. 



THE following is an abstract of the recent address of Mr. Hopkins, 

 President of the Geological Society of England The author first con- 

 siders the influence on the earth's superficial temperature of a central 

 heat, supposed to be the remains of a former and very much greater 

 heat which has been gradually diminishing during some indefinite 

 period of time. The effect on the superficial temperature due to this 

 cause may have been formerly of any amount, but is now reduced to 

 within one-twentieth of a degree of Fahrenheit, of that ultimate limit 

 to which it would be reduced in an indefinite period of time, suppos- 

 ing the external conditions under which the earth is now placed, such 

 as the amount of radiation from the sun and stars, and the state of the 

 atmosphere, to remain as at present. Poisson has calculated that it 

 would: require 100,000 millions of years to reduce the present temper- 

 ature by about one-fortieth of a degree of Fahrenheit. It is probable, 

 therefore, that many millions of years must have elapsed since the 

 central heat can have elevated the earth's superficial temperature by 

 a single degree. The author also explained that any very sensible 

 increase of superficial temperature from this cause must have been 

 attended with an exceedingly rapid rate of increase of the internal 

 temperature in descending below the earth's surface. It is only, how- 

 ever, to the more remote geological periods that we can refer for any 

 very sensible change in the climatal conditions of our globe due to this 

 cause. Such changes, also, must manifestly be continually from a 

 higher to a lower temperature ; and, therefore, we must appeal to some 

 other cause to account for such oscillations of temperature as those of 

 the glacial period. Poisson suggested that the present internal tem- 

 perature of our globe might not be due to its primitive heat, but to the 

 fact of the solar system having passed through some region of stellar 

 space of which the temperature, owing to stellar radiation, is much 

 greater than that in which it is now placed. Without professing to 

 say how far this cause may have influenced the climatal conditions of 

 the earth at former remote periods, the author shows that, reasoning 

 from all we know respecting the relative positions of the stars and the 

 probable motion of the solar system, this cause cannot have produced 

 a change so great as that which must have taken place during the gla- 

 cial epoch, at a time so recent as we have reason to believe that epoch 

 to have been. The author next proceeds to examine the effects of 

 changes in the disposition of land and sea, and of the consequent 

 changes in the direction of ocean currents. The map of isothermal 

 lines, recently published by MM. Humboldt and Dove, enables us to 

 estimate the influence of the existing configuration of land and sea, 

 and that of currents superinduced thereby, and thus we are enabled 

 to estimate approximately the effects of like causes in different hypo- 

 thetical cases. The isothermal lines have thus been constructed by 

 the author for the following cases : 1. When the progress of the gulf 

 stream into the North Sea is supposed to be intercepted by land con- 



