iv PREFACE. 



periods, and is resumed in connection with the recent glacial period, 

 where localization and periodicity, together with many details, are 

 best expressed. This recurrent treatment is intended to relieve, in 

 some measure, the stress of treatment of these intricate themes at 

 any one point, but more especially to combine dynamical discussion 

 with the phenomena which it is to explain, as these phenomena unfold 

 themselves stage by stage. This has been done in the belief that such 

 discussions often have their most obvious force only in such relations, 

 and in the further belief that the definite development of a historical 

 problem in its natural associations is an advantageous antecedent 

 to the discussion of the dynamic agencies that constitute its elucidation. 

 It has been thought that this association of phenomena with their causes 

 is important enough to justify even some reiteration of causes or rela- 

 tions, when these are essential to firm basal conceptions. 



In harmony with this recurrent recognition of the dynamical ele- 

 ment in the physical evolution of the earth, special effort has been 

 made to give to the evolution of the successive phases of the earth's 

 inhabitants their appropriate dynamic and physiographic relation- 

 ships. In addition to the more familiar relations of life to its environ- 

 ment, the special function of the epicontinental seas and their oscil- 

 lations has received emphasis, because these have been the chief media 

 through which the more legible part of the geologic record has been 

 made. As a corollary of this, the expansions and contractions of the 

 land, on the one hand, and of the epicontinental seas, on the other, 

 and their contrasted influences on the life of the land and of the shal- 

 low seas, respectively, have been given unusual prominence. 



Special emphasis is also laid on the features of the ancient lands 

 wherever the data permit. The sources of the sediments, and the 

 modes and conditions of their derivation, at all stages, are regarded 

 as equally important with the sediments themselves, and often more 

 significant of vital conditions. Base-level states, on the one hand, 

 and states of much relief, on the other, are recognized as influential 

 factors in determining not only the character of the deposits, but the 

 evolution of life on both land and sea. Terrestrial deposits, as dis- 

 tinguished from marine deposits, are recognized in many periods, 

 particularly in the Devonian, Carboniferous, Permian, Triassic, Coman- 

 chean, and Tertiary, and in the last, notably in the Lafayette. The 

 terrestrial deposits so recognized are not merely lacustrine, or even 



