Change of Climate during Geological Epochs. 133 



every kind of plant, even the coarse grass which covers the whole 

 surface of the land, becomes converted into this substance"*, 



From the evidence of geology we may reasonably infer that 

 were the difference between our summer and winter temperature 

 nearly annihilated, and were we to enjoy an equable climate 

 equal to, or perhaps a little above the present mean annual 

 temperature of our island, we should then have a climate similar 

 to what prevailed during the Carboniferous epoch. 



But we have already seen that such must have been the cha- 

 racter of our climate at the time that the excentricity of the 

 earth's orbit was at a maximum, and winter occurred when the 

 earth was in the perihelion of its orbit. For, as we have already 

 shown, the earth would in such a case be 14,753,834 miles 

 nearer to the sun in winter than in summer. This enormous 

 difference would almost extinguish the difference between sum- 

 mer and winter temperature. The almost if not entire absence 

 of ice and snow, resulting from this condition of things, would 

 probably tend to raise the mean annual temperature of the 

 climate higher than it is at present. 



The Climate of the Glacial and other Cold Epochs. 

 In this country the greater portion of the moisture of the air 

 is precipitated in the form of rain, and what happens to fall 

 during winter as snow disappears in the course of a few weeks 

 at most. But were the winter temperature very much reduced, 

 it is obvious that what now falls during that season as rain, 

 would then fall as snow. Under such circumstances it would 

 be very doubtful whether the heat of summer would be sufficient 

 to melt the snow of winter. Whether this would be the case or 

 not would depend upon the character of the summer. Under a 

 cloudless sky, the direct rays of the summer-sun would, in our 

 latitude, be more than sufficient to remove the winter's accu- 

 mulation of ice and snow. But if from thick fogs or an overcast 

 sky the direct rays of the sun were prevented from penetrating 

 to the earth, the heat of summer would not in such a case be 

 sufficient to remove the snow and ice; and the formation of 

 glaciers would be the inevitable result. Some may at first sight 

 suppose that the rain of summer would be sufficient of itself to 

 melt the snow of winter, but such would not be the case \ for 

 it takes nearly eight tons of water at 50° F. to melt one ton of 

 snow, even when the latter is already in a thawing condition. 

 It is therefore perfectly evident that all the rain of summer 

 would not be sufficient to melt more than one-eighth part of the 

 snow of winter f. Prof. Forbes found that not more than one- 



* Journal of Researches, chap, xiii, 

 t Phil. Mag. for May 1864. 



