PLAN ETESIMAL THEORIES OF THE EARTH'S ORIGIN. 



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Coming to the question whether the accumulation of so large 

 a body as the earth took place without its becoming intensely 

 hot and molten, somewhat like the sun, we have first the 

 observations and theories of geology to aid in giving an answer, 

 and these may be advantageously supplemented by the 

 physiographic features of our satellite, the moon. It has been 

 long held by geologists that the downward increase of heat in 

 the earth's crust, present volcanoes, the widely distributed 

 evidences of ancient volcanic action, and thermal metamor- 

 phism of great rock formations, indicate an internal 

 temperature which must fuse any known rocks, unless 

 they are prevented from this by overlying pressure. 

 The new hypothesis of Chamberlin accounts for vulcanism, 

 and for all that we know of the earth's internal heat, fully 

 as well as the Laplacian hypothesis of condensation of 

 an intensely hot gaseous nebula, while it better accords 

 with the physical and dynamic relations of the planets and 

 sun. 



If our inquiry be turned to the moon, we see a most 

 wonderful record, as it is generally regarded, of extinct volcanic 

 action, implying a formerly very hot and probably almost 

 wholly molten state of that globe, which has a little more than 

 one-fourth the diameter of the earth. These two companion 

 globes were doubtless accumulated similarly. The moon, after 

 acquiring its present size, had multitudes of volcanoes which 

 left round craters, or parts of their crater rims, of varying 

 dimensions from those at the limit of telescopic vision up to 

 one with a diameter of about 800 miles, or nearly four-fifths of 

 the moon's radius. So great a lake or sea of molten rock, 

 similar to the calderas of the Hawaiian volcanoes, but of 

 vastly larger area, whose crater rim is partially preserved in 

 the lunar Carpathian-Apennine-Caucasus chain of mountains, 

 could only exist when much of the interior of the moon was 

 melted. It seems possible and indeed probable, therefore, that 

 the earth, whether formed as supposed by the old or the new 

 nebular hypotheses, was nearly or quite all melted during a 

 considerable part of the time of its accumulation. The 

 planets undoubtedly tended in some degree toward the 

 same intensely hot condition, which is reached by the 

 sun and stars in the concentration of originally nebulous 

 matter. 



But another explanation of the origin of the very abundant 

 small and large crateriform features of the moon has been 

 advocated by G. K. Gilbert, of the United States Geological 



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