Febeuaby 1, 1907] 



SCIENCE 



171 



but he conceived of it as a 'plain of marine 

 denudation,' and that phrase holds its 

 place to-day in English writings. With 

 genuine British loyalty and conservatism, 

 Sir Archibald Geikie retains the phrase 

 even in the latest edition of his ' Text-book, ' 

 in explanation of table-lands of erosion; 

 though, even in his earliest edition, he de- 

 clared that, in the production of plains of 

 marine denudation, "the sea has really had 

 less to do than the meteoric agents. A 

 plain of marine denudation is that sea-level 

 to which a mass of land has been reduced 

 mainly by the subaerial forces." He at- 

 tributes to ocean waves and currents only 

 'the last touches in the long process of 

 sculpturing.' Elsewhere Geikie bears em- 

 phatic testimony to the influence of Amer- 

 ican geologists, in the words : ' ' Unquestion- 

 ably the most effective support to Hutton's 

 teaching has been given by the geologists 

 of the United States, who, among the com- 

 paratively undisturbed strata of the west- 

 ern territories, have demonstrated, by 

 proofs which the most sceptical must re- 

 ceive, the potency of denudation in the 

 production of the topography of the land. ' ' 

 It is, indeed, marvelous with what pro- 

 phetic vision Hutton and Playfair con- 

 ceived some of those ideas of river action 

 which the geologic world in general learned 

 only from the work of American geologists 

 in the Cordilleran region. On this subject, 

 those two Scotchmen were a half-century 

 in advance of their time. 



That western land has taught us not only 

 to recognize the fact of subaerial denu- 

 dation, but also to formulate its methods. 

 In Powell's ' Exploration of the Colorado 

 Eiver,' he distinguished rivers as conse- 

 quent, antecedent, and superimposed. 

 Davis has carried the analysis somewhat 

 further, giving us subsequent and obse- 

 quent rivers. Powell formulated the doe- 

 trine of base-levels; Davis has given the 

 conception greater accuracy and consist- 



ency by distinguishing base-level from 

 profile of equilibrium. The base-level of 

 a district is a portion of the ideal sphe- 

 roidal surface of the earth. Disregarding 

 the curvature of the earth's surface, base- 

 level may be represented in profile as a 

 straight line; profile of equilibrium as a 

 curve, concave upward, tangent to base- 

 level at the mouth of the river, gradually 

 approaching base-level with the progressive 

 denudation of the country, but never quite 

 reaching it except at the point of tangency. 

 To Davis also we owe the full development 

 of the conceptions of youth and age in 

 river valleys and in drainage systems, and 

 of cycles of erosion ending in the forma- 

 tion of peneplains. We have learned to 

 search in every rugged mountain region 

 for remnants of ancient peneplains. We 

 have learned, in general, that geological 

 history is to be read, not only in deposits, 

 but also in erosion forms. 



v. TERTIARY MAMMALS AND THE DOCTRINE 

 OF EVOLUTION 



Half a century ago the exploring ex- 

 peditions connected with the Smithsonian 

 Institution began to collect fossils from the 

 Tertiary deposits of the western plains. 

 Later the work was followed up by the 

 Geological Surveys under the auspices of 

 the national government, and by numerous 

 private expeditions under the auspices of 

 universities, scientific associations and in- 

 dividuals. Over those western plains were 

 found to stretch vast continental deposits, 

 certainly not all of lacustrine origin, as at 

 first reported, but in part piedmont allu- 

 vial formations, in part eolian deposits, 

 and, in limited areas, deposits of volcanic 

 dust. These continental deposits of the 

 western plains yielded in unparalleled rich- 

 ness mammalian fossils, which have been 

 studied by Leidy, Marsh, Cope, Osborn, 

 Scott, Wortman, and others. No other 

 single series of discoveries has been so 



