INTRODUCTION 153 



almost universal law there is but one exception, namely, the internal 

 forces of the earth. 



"Thus, then, all geological agencies are primarily divided into two groups. 

 In the one group come atmospheric, aqueous, and organic agencies, together 

 with all other terrestrial phenomena which constitute the material of science ; 

 in the other group, igneous agencies and their phenomena alone. The forces 

 in the one group are exterior ; in the other, interior ; in the one sun-derived ; 

 in the other, earth-derived. The one forms, the other sculptures, the earth's 

 features. . . . The general effect of the one is to increase the inequalities of 

 the earth's surface, the other to decrease and finally to destroy them. . . . 

 All that constitutes physical geography at any geological time is determined 

 l)y the state of balance between these two eternally antagonistic forces." 



It is now well established that the geologic history of the earth is 

 •cyclic in its nature, cyclic in that the periods have (1) long intermediate 

 times when the lithosphere undergoes peneplanation and warping move- 

 ments comprehended under the term epeiro genie, and (2) shorter closing 

 epochs when the earth^s outer shell is locally folded into mountains, the 

 vrogenic times. During the intermediate times, organic evolution is al- 

 most static (biostatic), but is quickened in the cooler and drier orogenic 

 'epochs. The times of lesser crustal folding occur toward the close of the 

 periods, and when the diastrophism is world-wide, as toward the close of 

 the eras, then the. life of the earth passes through critical or revolutionary 

 times, with quickened evolution and vanishing of the overspecialized 

 stocks. 



The cause of the cyclic changes in the surface of the earth is to be 

 sought in the variable tensions in, and the elasticity of, the lithosphere. 

 These are brought about by radial shrinkage, or the contraction energies. 

 The tensions are of slow accumulation during the epeirogenic times, and 

 they give way to crustal movements during the orogenic epochs of com- 

 pensation. 



Rise of the Theory of Geosynclines 

 general discussion 



The unraveling of American geology began in earnest with the or- 

 ganization of the State Survey of Massachusetts in 1830, and in 1836 

 that of New York, the State destined to be the court of last resort in 

 Paleozoic stratigraphy. Merrill tells us in his comprehensive "Contri- 

 butions to the history of American geology" that during the decade 1830- 

 1839 official surveys were organized by no fewer than fifteen States, and 

 national ones carried on by Owen and Featherstonhaugh in the Missis- 

 sippi Valley. During the next twenty years eleven other State surveys 



