PREFACE. vii 



currency in America. The Lower Cretaceous is also recognized as a 

 separate system under the name Comanchean. The reasons for these 

 departures from current classification are set forth in the text. Some 

 suggestions of further modification of the current classification are 

 made, but not adopted, and in connection with them the criteria of 

 classification are set forth. 



Some periods have received fuller treatment than others for special 

 reasons, usually for the sake of treating new features with some ampli- 

 tude when they first present themselves in forms advantageous for 

 exposition. The Cambrian period, for example, affords the first unmu- 

 tilated record of a great sedimentary fossil-bearing system, and hence 

 receives an elaboration that becomes unnecessary with its successors. 

 So the Devonian period furnishes the first impressive expression of 

 provincial life-evolution on a large scale; the Carboniferous, the first 

 declared plant evolution; the Permian, the first well-deciphered gla- 

 ciation; the Pleistocene, the immediate preparation for the human 

 period, and so the text of these periods has been swollen by these special 

 themes. 



As in the previous volume, the endeavor has been made to keep 

 the discussion as free from technicalities as practicable, and to render 

 the matter readable. Certain of the more complicated problems of 

 internal and atmospheric dynamics render this well-nigh unattainable, 

 as frankly confessed by the relegation of certain portions to small 

 type. The description of the many complexities of the geographic 

 distribution and of the composition of the various formations cannot 

 be wholly freed from tediousness to the general reader; but they are 

 so essentially serviceable for local and special studies, and even for 

 a concrete general conception of the variety of concurrent geological 

 processes, that their admission is regarded as indispensable. 



The three volumes are designed to furnish the basis for a year's 

 work in the last part of the college course, or in the early part of a 

 graduate course. It may seem at first thought that the amount of 

 matter is rather large for this, but the educational saw, that, in things 

 historic, nineteen pages of flesh-and-blood are an easier assignment 

 than nine pages of bare-bones, is believed to have an application to 

 geologic history. By judicious selection of material to be presented 

 and omitted, the volumes will be found useful for briefer courses, and 

 by the use of the numerous references to the discussions of special 



