598 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



gion. But there is a rapidly growing class of geologists who believo 

 that there is a natural system that, at least in its broader lines, is of 

 general applicability. Chamberlin and Salisbury (Geology, vol. 3, 1906, 

 pages 192-194) have expressed themselves very clearly on this matter. 

 As their views are in essential accord with my own, I shall take the 

 liberty to quote them: 



"We believe that there is a natural basis of time division, that it is recorded 

 dynamically in the profounder changes of the earth's history, and that its 

 basis is world-wide in its applicability. It is expressed in interruptions of the 

 course of the earth's history. It can hardly take account of all local details, 

 and can not be applied with minuteness to all localities, since geological history 

 is necessarily continuous. But even a continuous history has its times and 

 seasons, and the pulsations of history are the natural basis for its divisions. 



"In our view, the fundamental basis for geologic time divisions has its seat 

 in the heart of the earth. Whenever the accumulated stresses within the body 

 of the earth over match its effective rigidity, a readjustment takes place. The 

 deformative movements begin, for reasons previously set forth, with a depres- 

 sion of the bottoms of the oceanic basins, by which their capacity is increased. 

 The epicontinental waters are correspondingly withdrawn into them. The 

 effect of this is practically universal, and all continents are affected in a 

 similar way and simultaneously. This is the reason why the classification of 

 one continent is also applicable, in its larger features, to another, though the 

 configuration of each individual continent modifies the result of the change, so 

 far as that continent is concerned. 



. . . "In these deformative movements, therefore, there seems to us to be 

 a universal, simultaneous, and fundamental 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 appli- 

 cation in detail to the varied rock formations in all quarters of the globe. 



. . . "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) baseleveling, (2) sea -filling, (3) continental 

 creep, and (4) sea transgression; but it requires little prophetic vision to see a 

 probable demonstration of it in the near future. Subordinate 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 irre- 

 ducible to order. 



. . . "In applying a classification based on body deformation, some regard 

 must be had to the fact that while sea withdrawal, as the result of increased 

 capacity of the sea basins, is simultaneous the world over, continental defor- 

 mations and crustal foldings are more local and less nearly synchronous, for 

 there is no agency to combine and equalize their effects as in the case of the 

 basins. Continental deformations must be employed in the classification with 

 some latitude, and correlations based on them can not be expected to have an 

 equally high order of exactness. Local advances and retreats of the sea due 

 to local warpings must be eliminated or neglected, in a general classification, 

 for the reason that tliey are local." 



