306 MA ^ AND THE GLACIAL PERIOD. 



period, which, in turn, was dispelled by fresh showers of 

 meteorites upon the sun, sufficient to produce the amelio- 

 ration of climate which we experience at the present time. 



As intimated, this theory is closely allied to the pre- 

 ceding, the principal difference being that it limits the 

 eifects of the supposed cause to the solar system, and looks 

 to our sun as the varying source of heat-supply. It has 

 the advantage over that, however, of possessing a more 

 tangible vera causa. Meteorites, asteroids, and comets 

 are known to be within this system, and have occasional 

 collisions with other members of it. But the principal 

 objection urged against the preceding theory applies here, 

 also, with equal force. The accumulations of ice during 

 the Glacial period were not determined by latitude. In 

 North America the centre of accumulation was south of 

 the Arctic Circle — a fact which points clearly enough to 

 some other cause than that of a general lowering of the 

 temperature exterior to the earth. 



The same objections would bear against the theory 

 ably set forth by Mr. Sereno E. Bishop, of Honolulu, 

 which, in substance, is that there may be considerable 

 variability in the sun's emission of heat, owing to fluctua- 

 tions in the rate of the shrinkage of its diameter, brought 

 about by the unequal struggle between the diminishing 

 amount of heat in the interior and the increasing force of 

 the gravitation of its particles, and by the changes in the 

 enveloping atmosphere of the sun, which, like an enswath- 

 ing blanket, arrests a large portion of the radiant heat 

 from the nucleus, and is itself evidently subject to vio- 

 lent movements, some of which seem to carry it down to 

 the sun's interior. Unknown electrical forces, he thinks, 

 may also combine to add an element of variability. These 

 supposed changes may be compared to those which take 

 place upon the surface of the earth when, at irregular 

 intervals, immense sheets of lava, like those upon the 

 Pacific coast of North America, are exuded in a compara- 



