304 THOMAS CHROWDER CHAMBERLIN 



the precision of correlation by unconformities has more apparent 

 than real value, for the different parts of the same unconformity vary 

 much in time. All distant correlations involve some measure of 

 inexactness, and the more frankly it is made obvious, the less its 

 liability to mislead. 



Correlation by general diastrophic movements takes cognizance 

 of four stages: (i) the stages of climacteric base-leveling and sea- 

 transgression, (2) the stages of retreat which are the first stages of 

 diastrophic movement after the quiescent period, (3) the stages of 

 climacteric diastrophism and of greatest sea-retreat, and (4) the 

 stages of early quiescence, progressive degradation, and sea-advance. 



(i) The characteristics of the climacteric stage of base-leveling 

 and sea-transgression need little further characterization here, for the 

 function of base-levels is known to all American geologists and the 

 function of great sea-transgression to every stratigrapher and paleon- 

 tologist. We have in base-leveling conjoined with sea-transgression, 

 just that combination of agencies which is competent to develop the 

 broad epicontinental seas of nearly uniform depth requisite for an 

 expansional evolution of shallow-water life. At the same time, it 

 furnishes broad pathways around and across the continental surfaces 

 for wide migrations and the comminglings that lead to cosmopolitan 

 faunas of the shallow-water type. 



(2) The stages of initial diastrophism and sea-retreat find their 

 criteria in the deposits that spring from an increased erosion of the 

 deep soil-mantles accumulated in the base-level period, in the effects 

 of increasing turbidity, in the lessening areas suitable for the shallow- 

 water life, and in the limitation of migration. 



(3) The climacteric stages of diastrophism are marked by the 

 stress of restrictional evolution among the shallow- water species; by 

 increased clastic deposition in land basins, on low slopes, and on sea 

 borders, by great land extension, but often, perhaps dominantly, by 

 diversity of land surface and by liability to climatic severities and 

 diversification. Areally, land life is favored, but it is hampered by 

 the climatic and topographic diversities, and these may prove graver 

 obstacles to migration and intermingling than even the tongues of 

 sea that previously traversed the land surface. Correlation by 

 glaciation in these stages»is likely to prove a valuable adjunct, but we 



