PLANETJ5SIMAL THEORIES OF THE EARTH'S OKIGIN. 199 



then, as now, an effective cushion, checking the speed of the plane- 

 tesimals and partially dissipating them, and, in the second place, 

 the early organisms were probably all aquatic and were further 

 protected by their water covering. . . . 



" So soon as plants and animals had come into action, all the 

 great factors potential in the earth's physical evolution were in 



p la y- 



" By hypothesis, volcanic action only began some time after the 

 beginning of the earth's growth, for it was delayed (1) by the lack of 

 sufficient compression in the central parts to give the requisite heat, 

 and (2) by the time required for this central heat to move out to 

 zones of less pressure, where it would suffice to melt the more 

 fusible constituents. But, once begun, it is supposed to have 

 gradually increased in actual and in relative importance until it 

 reached its climax. This obviously came much later than the 

 climax of growth, for it was dependent on the growth to give the 

 increased compression from which arose the central heat on which 

 the vulcanism depended. . . . 



"The formations of this period of volcanic dominance, with very 

 subordinate clastic accompaniment, are regarded as constituting 

 the Archean complex, though perhaps only the later portions of the 

 great volcanic series are represented by the known Archean." 



To give a satisfactory statement of Professor Chamberlin's 

 exceedingly interesting and elaborate theory has thus required 

 very large quotation from his recent publication of it. Only by 

 such direct presentation of his work in his own words could 

 justice be done to this new nebular theory, to which this 

 eminent glacialist was primarily led by his endeavours to 

 explain the causes of the Ice Age, and of its several waxing 

 and waning stages, by periodic changes in the content of 

 carbon dioxide in the atmosphere. Having been an assistant 

 under his direction on the United States Geological Survey 

 during seven years in my work on the Glacial Lake Agassiz, it 

 is with great pleasure and pride that T can claim for him and 

 for America the distinguished honour of having developed this 

 great theory of the origin of the earth. It will certainly 

 introduce into geology and geophysics many new and fruitful 

 methods of observation and research. Indeed, nearly all the 

 great fields of theoretical geology now require renewed 

 investigation, by which the planetesimal hypothesis shall be 

 tested. 



An earlier address by Professor Chamberlin, partially setting 

 forth his studies in this direction, was given before the 

 Geological Society of America, in Washington, D.C., on 

 January 1, 1903, entitled "Origin of Ocean Basins on the 



o 



