A CENTURY OF GEOLOGY. 549 



a lost interval — certain leaves missing from the book of time. 

 And if the unconformity be widespread, the lost interval is corre- 

 spondingly great. It is therefore probable that change of species 

 went on slowly and uniformly all the time, although not recorded 

 at that place. Intermediate strata may be and often are fomnd 

 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 duty of the geologist to gather and arrange 

 them in proper order, so that the record may read continuously. 



This is the uniformitarian view, and is undoubtedly far truer 

 than the catastrophic. But the objection to it is that in the case 

 of very widespread 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 comparative quiet (not fixedness) during which 

 evolutionary changes were slow and regular, and periods of revolu- 

 tion 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 by 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 read- 

 justments 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 min- 

 glings 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 

 dominant types, and these, in their turn, would compel new adjust- 

 ment 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 re-established and a new period of com- 

 parative stability and prosperity commenced. 



This "sdew is a complete reconciliation of catastrophism and 

 uniformitarianism, and is far more rational than either extreme. 



Critical Periods in the History of the Earth. — Such periods of 

 rapid change may well be called critical periods or revolutions. 

 They are marked by several characteristics: (1) By Avidespread 

 oscillations of the earth's crust, and therefore by almost universal 

 unconformities. (2) By widespread changes of physical geography, 



