THE EOCENE PERIOD. 193 



of the means of intercontinetal correlation. Second only to this in importance 

 is the influence on terrestrial life through the connections and disconnections 

 that control migration. Springing from the same deformative movements are 

 geographic and topographic changes, affecting not only the land, but also the 

 sea currents. These changes affect the climate directly, and by accelerating 

 or retarding the chemical reactions between the atmosphere, hydrosphere, and 

 lithosphere, affect the constitution of both air and sea, and thus indirectly influ- 

 ence the environment of life, and through it, its evolution. In these deformative 

 movements, therefore, there seems to us to be a universal, simultaneous, and fun- 

 damental basis for the subdivision of the earth's history. It is all the more 

 effective and applicable, because it controls the progress of life, which furnishes 

 the most available criteria for its application in detail to the varied rock forma- 

 tions in all quarters of the globe. 



The main outstanding question relative to this classification is whether 

 the great deformative movements are periodic rather than continuous, and 

 cooperative rather than compensatory. This can only be settled by compre- 

 hensive investigation the world over; but the rapidly accumulating evidence 

 of great base-leveling periods, which require essential freedom from serious 

 body deformation as a necessary condition, has a trenchant bearing on 

 this question. So do the more familiar evidences of great sea transgressions, 

 which may best be interpreted as the consequence of general base-leveling 

 and concurrent sea-filling, abetted by continental creep during a long stage 

 of body quiescence. It is too early to affirm, dogmatically, the dominance in 

 the history of the earth of great deformative movements, separated by long 

 intervals of essential quiet, attended by (1) base-leveling, (2) sea-filling, (3) con- 

 tinental creep, and (4) sea transgression; but it requires little pro- 

 phetic vision to see a probable demonstration of it in the near future. Sub- 

 ordinate to these grander features of historical progress, there are innumerable 

 minor ones, some of which appear to be rhythmical and systematic, and some 

 irregular and irreducible to order. These give rise to the local epochs and epi- 

 sodes of earth-history, for which strict intercontinental correlation cannot be 

 hoped, and which must be neglected in the general history as but the individuali- 

 ties of the various provinces. 



The periods which have been recognized in the Paleozoic and Mesozoic, 

 chiefly on the basis of European and American phenomena, seem to us likely 

 to stand for the whole world, with such emendations as shall come with widening 

 knowledge. 



The classification of the Cenozoic is more hampered by the artificiality of 

 its names, by the intricacy of its details, and by the (as yet) imperfect appli- 

 cation of the newer modes of investigating and interpreting the phenomena of 

 the geology of the land, as distinguished from the older branch, the geology of the 

 sea. A large part of the known deposits of the Tertiary are non-marine. They 

 have been interpreted as lacustrine, and the areas of their deposition as lake 

 basins. The Tertiary has even been called the age of lakes. Certain topo- 

 graphic interpretations are necessary to provide the requisite basins, and this 



