METHODS UNDERLYING A DETERMINED CHRONOLOGY 503 



succession indicating sea withdrawals and erosion intervals, (4) by abrupt 

 changes from marine to continental and from land to sea deposits or from 

 limestones to sandstones, and (5) by overlapping or transgressing for- 

 mations. 



The larger crustal deformations are periodic in appearance and their 

 visible areas of movements are now in one continent and now in another, 

 and it is this periodicity that conditions paleogeography and quickens 

 evolution. Each one of these active and decisive movements is of long 

 duration, and their major work is confined to the marginal areas of the 

 continent. At the same time the oceanic basins are made either deeper 

 or larger, or both. This simultaneous movement of the oceanic bottoms 

 and the continental margins, is proved by the fact that the crustal defor- 

 mations occur during the emergent and closing epochs of the periods. 

 This is true not only for the continent deformed, but for other land 

 masses as well that have not moved at all, for the strand-lines of the 

 latter have also been lowered in consequence of the oceanic enlargement. 



The long-enduring middle portion of the periods is marked by relative 

 crustal stability, peneplanation of the continents, and maximum sea in- 

 vasion, brought about in the main through the unloading of the con- 

 tinental protuberances into the oceanic basins. On the other hand, the 

 first epoch of each period exhibits much crustal warping and marked 

 erosion. The lands then warp more or less along predetermined lines, 

 due to compensating deep internal adjustments following the major 

 movement and to the reestablishment of the isostatic balance that has 

 been altered by the deformation, by the sea invasion, and by the unload- 

 ing of protuberant land areas into the loading seas. During the closing* 

 epoch of the periods there is a renewal of crustal unrest, seen in the 

 vanishing of the inland seas, and finally ending in another marked crustal 

 deformation and in more or less complete withdrawal of the oceanic 

 overlaps. 



There is a certain amount of rhythm in these periodic changes in the 

 face of the earth, and it is this meter that permits of grouping the for- 

 mations into the systems. Each long-enduring submergence with the 

 following emergence is seemingly the natural basis for the delimiting of 

 the periods. Among these periodic movements some are far more in- 

 tense and of greater geographic extent than others, and at such times 

 mountain ranges are thrown up in most of the continents. These are the 

 diastrophic grand cycles, the critical periods or revolutions in the history 

 of the earth, and they bind, as it were, the periods or disturbances into 

 the eras. 



