280 A CENTUEY OF GEOLOGY. 



breaks that gave ri.se to the l)elief in the oeeurrence of violent catas- 

 trophes and corresponding wholesale exterminations and re-creations 

 of faunas and lioras. 



It is evident, however, on a little reliection, that every such uncon- 

 formity indicates a land period at the place observed, and therefore a 

 time unrecorded in strata and fossils at that place — i. e., a lost interval — 

 certain leaves missing from the book of time. And if the unconformity 

 be widespread, the lost interval is correspondingly great. It is there- 

 fore probable that change of species went on slowl}^ and uniforml}^ ail 

 the time, although not recorded at that place. Intermediate strata 

 may be and often are found elsewhere, and the supposed lost interval 

 filled. The record was continuous and the changes uniform, but the 

 record is not all found in one place. The leaves of the book of time 

 are scattered here and there, and it is the dut}" of the geologist to gather 

 and arrange them in proper order, so that the record may read con- 

 tinuously. 



This is the uniformitarian view, and is undou])tedly far truer than the 

 catastrophic. I^ut the objection to it is that in the case of very wide- 

 spread unconformities, such as occurred several times in the history of 

 the earth, the changes of organisms are so great that if the rate of 

 change was uniform the lost interval must have been equal to all 

 the rest of the history put together. Therefore we are compelled to 

 admit that in the history of the earth there have been periods of com- 

 parative quiet (not fixedness) during which evolutionary changes were 

 slow and regular, and periods of revolution during which the changes 

 were much more rapid, but not catastrophic. This is exactly what we 

 ought to expect on the idea of gradual evolution of earth forms b}'' 

 secular cooling, for in the gradual contraction of the earth there must 

 come times of general readjustment of the crust to the shrinking 

 nucleus. These readjustments would cause great changes in physical 

 geography and climate, and corresponding rapid changes in organic 

 forms. In addition to this, the changes in physical geography and 

 climate would cause extensive migrations of species, and therefore 

 minglings of faunas and floras, severer struggles of competing forms, 

 and more rapid advance in the steps of evolution. Among these 

 changes of organic forms there would arise and have arisen new domi- 

 nant types, and these, in their turn, would compel new adjustment of 

 relations and still further hasten the steps of evolution. Such changes, 

 whether geographic, or climatic, or organic, would not be simultaneous 

 all over the earth, but propagated from place to place, until quiet was 

 reestablished and a new period of comparative stability and prosperity 

 commenced. 



This view is a complete reconciliation of catastrophism and uniformi- 

 tarianism, and is far more rational than either extreme. 



