256 



SCIENCE 



[N. S. Vol. XXXI. Xo. 790 



only the continental masses adjacent to 

 that basin or basins. The value of periodic 

 activity in correlation is thus conditioned 

 by the regional distribution of diastro- 

 phism. 



Continents bordering on one and the 

 same dynamic province have usiially been 

 disturbed during one and the same period. 

 Opposite sides of one and the same con- 

 tinent, however, bordering on different 

 dynamic provinces, do not exhibit similar 

 conditions of disturbance at the same time. 



"Western Europe and eastern North 

 America, for instance, exhibit parallel dias- 

 trophic histories, peculiar to the North At- 

 lantic basin, but the diastrophic histories of 

 the Atlantic and Pacific sides of North 

 America do not run parallel. 



Any great period of diastrophic activity, 

 though relatively short as compared with 

 an era of inactivity, is very long in com- 

 parison with any particular movement 

 incidental to its own development. Thus 

 Pennsylvanian and Permian diastrophism 

 had a long history before the folding of 

 strata occurred in the Appalachian trough. 

 Such an incident of folding, or of any 

 erogenic growth whatever, is locally con- 

 ditioned. It results from local structures 

 and localized pressures. The time of dis- 

 turbance depends upon the local position 

 of the district in relation to the source of 

 disturbing stresses, and is peculiar to the 

 district. The phenomena of folding, or of 

 orogenic growth, therefore, do not afford 

 criteria for correlation beyond the area of 

 special conditions. We may not safely 

 correlate the displacement of the Appa- 

 lachian zone with the disturbance of the 

 Carboniferous in England, for instance, 

 although both events occurred during the 

 same diastrophic period and belong to one 

 and the same dynamic province. 



Within any orogenic district, the epoch 

 of general disturbance is a principal ele- 



ment of classification. It is set apart, it is 

 unavoidable. It must be recognized, and it 

 commonly separates major divisions. 



This stated, it is none the less of the first 

 importance to insist on the local character 

 of the criteria. A district of orogenic dis- 

 turbance is sharply limited by mechanical 

 conditions. Across the line exist other 

 conditions, which are inconsistent with 

 orogeny and its manifestations. The cri- 

 teria of correlation by orogeny fail, there- 

 fore, beyond the line. Disturbance and 

 quiet, erosion and continuous deposition, 

 unconformity and conformity, have de- 

 veloped simultaneously in immediately ad- 

 jacent districts many times. 



In strong contrast to continental and 

 orogenic movements, which develop sub- 

 aerially, are those subsidences which occur 

 beneath the oceans. Though they also are 

 more or less local, their effects are prac- 

 tically world-wide, for any submarine 

 movement modifies the capacity of ocean 

 basins and changes the position of sea 

 level on all coasts. No other phenomenon 

 is so nearly simultaneous. 



If any one of the confluent ocean basins 

 be deepened the sea level datum about all 

 lands must be lowered. The effect may 

 not be evident in the exceptional case that 

 any land subsides by a due amount; but 

 the exceptional ease is not likely to mask 

 the general effect of a universal ebb tide. 

 Shallowed and reduced epicontinental seas, 

 low islands and low coastal plains mantled 

 with the latest sediments, slight erosion 

 and unconformity without disturbance of 

 the strata, constituting a general condition 

 of continents, these are the characteristic 

 phenomena of such an ebb. They distin- 

 guish the middle Ordovician of regions as 

 remote as eastern North America, eastern 

 China and western Europe. They mark 

 also the passage from Cretaceous to Eocene. 



Suboceanic movements have no doubt 



