306 



PAST CLIMATES AND CLIMAXES. 



beds or formations separated by a time interval, while a similar section of the 

 adjacent and lower conformity shows the time interval as recorded in a bed 

 or formation derived from the erosion of the exposed portion of the formation 

 upon which it rests (fig. 24). 



Unconformity is, then, an invaluable indicator of deformation movements. 

 In the case of general body deformations, unconformity should be the rule. 

 Conversely, general or widespread unconformity is evidence of great deforma- 

 tion, and hence of a major cycle. Local unconformity indicates minor or 

 regional deformation, though the latter may be related to the body deforma- 

 tion or independent of it. In consequence, it is regularly associated with the 

 regional gradations which 

 are here regarded as typi- 

 cal cycles of erosion. In 

 any region the series of gen- 

 eral and local unconformi- 

 ties is an exact measure 

 of the alternation of defor- 

 mation and gradation, and 

 hence of generally conti- 

 nental and oceanic climates. 

 If the latter be checked by 

 the critical scrutiny of the 

 fossils of the land biota 

 found in the two uncon- 

 formable formations, and 

 especially by those found 

 in the conformable forma- 

 tion which results from the 

 erosion phase, we secure an 

 almost perfect record of the 

 sequence of deformation, 

 climate, vegetation, and 

 fauna. 



Correlation of climates.— 

 It is increasingly evident 

 that the changes in past 

 climates may have been caused either by deformation or by solar variations, 

 as well as by the interaction of the two. The problems of climatic change are 

 considered in some detail in the next section, and the general series of climatic 

 cycles is suggested. It will here suffice to point out the correspondence of 

 the major geological chmates with the periods of greatest body deformations. 

 The outstanding deformations are the pre-Cambrian, the Cretaceous-Eocene, 

 the Permian-Triassic, and the PHocene. All but the second of these were 

 followed by a period of maximum glaciation, and a marked differentiation of 

 climates, and the second was succeeded by a marked cooling. The first two 

 were followed by the evolution of a new land flora, and the last by a unique 

 shifting of vegetation climaxes. We are probably still in the period of read- 

 justment after the glaciation consequent upon Pliocene deformation, and the 

 question of the evolution of a new flora lies too near at hand to permit the 



Fig. 24. — Origin of unconformity. Upper figure shows 

 sedimentary rock, Al, lying upon older rock, AB. In 

 the middle figure, elevation of land or retreat of sea 

 has exposed layer Al to erosion, and the eroded mate- 

 rial has been deposited as a new layer, 1. In lower 

 figure, subsidence has caused the sea to return to its 

 original level, and a new sedimentary layer, 2, has 

 been deposited above Al and 1. This new formation 

 shows conformity with 1, but unconformity with Al 

 and AB. After Chamberlin and Salisbury. 



