vi PREFACE. 



evolution of life is made a subject of constant remark, with sugges- 

 tions as to biological, as well as physiographic, relationship and depend- 

 ence. 



In the biological, as in the physical parts, the unsolved problems 

 are frankly recognized, and the student is made a party to a common 

 desire, if not a common effort, to secure more light on dark subjects. 

 These volumes therefore attempt not merely to record the salient 

 facts of historical geology, and to suggest dynamic relations and inter- 

 pretations, but to indicate to rising geologists the need of further inves- 

 tigations, of more light, and of revised interpretations on not a few 

 phases of the earth's history. 



Perhaps the most radical departure of this work from the prece- 

 dents of its class lies in the larger emphasis laid on the hypotheses 

 of the origin of the earth, and especially in the introduction of a new 

 hypothesis of earth genesis, whose dynamic sequences depart widely 

 from familiar lines. This recognition of divergent hypotheses at the 

 very outset necessarily involves an effort to carry through the inter- 

 pretations of the whole history parallel systems of doctrine built on 

 the diverse hypotheses recognized. Especially is it incumbent to 

 try to carry out the logical sequences of the new hypothesis in its appli- 

 cation to the main problems that remain unsolved, particularly those 

 of deformation, vulcanism, and atmospheric states. Introduced con- 

 currently with the more familiar views, these newer modes of inter- 

 pretation form an alternative system running through the whole work. 

 It is believed that this will be at least stimulating, whether or not it 

 shall prove to be a step toward the final system of interpretation, 

 which future research alone will determine. 



A few new elements of geologic classification which have not usu- 

 ally received more than partial recognition in standard works are 

 herein given full recognition. In the class of time-divisions technic- 

 ally designated Eras, the Proterozoic, long since proposed by Irving, 

 has been adopted, and made to cover about the ground included under 

 the Algonkian of the U. S. Geological Survey. In other words, it is 

 made to include the group of chiefly sedimentary or meta-sedimentary 

 systems below the Cambrian and above the great terranes of chiefly 

 igneous or meta-igneous origin, referred to the Archeozoic. The Lower 

 Carboniferous, or Sub-carboniferous, is recognized as a distinct sys- 

 tem under the name Mississippian, which has already received some 



