478 CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 



GEOLOGY. 



On Critical Periods in the History of the Earth, and their relation 

 to Evolution; on the Quarternary as such a Period. 



BY JOSEPH LECONTE. 

 Read befoie the Kational Academy of Sciences, Apiil 18, 1877. 



In the series of rocks representing the history of the earth there occur 

 at different horizons uncomformities. In Biost cases these are not found at 

 the same horizon in diflPerent places ; but there are a few which seem to be 

 very general. Associated with these unconformities, as is well known, there 

 is nearly always a marked change^in the fossil species. The greatness of 

 this change is invariably in direct proportion to the generality of the un- 

 conformity. These general unconformities attended with very great changes 

 in organic forms are the natural boundaries of the great divisions of time, 

 and the less general unconformities attended with less sweeping change of 

 organic forms, of the subdivisions of time. 



The earlier geologists, under the influence of the then dominant idea of 

 frequent supernatural interference with the course of nature, imagined 

 that these unconformities marked the times of instantaneous cataclysm 

 which disturbed the rocks and destroyed all living things, sometimes locally,, 

 sometimes generally, and that these exterminations were followed by re- 

 creations of other and wholly different species at the beginning of the sub- 

 sequent period of tranquility. Novo, however, we believe that no such in- 

 stantaneous general exterminations and re-creations ever occurred. "We 

 know that unconformity simply indicates eroded land-surface, and there- 

 fore marks a period of time during which the observed place was land and 

 received no sediment; that two series of rocks unconformable to each 

 other denotes two periods of comparative quiet, during which the ob- 

 served place was sea-bottom, receiving sediment steadily, separated by a 

 period of oscillation producing increase and decrease of land, during which 

 the observed place was raised into land-surface, with or without crumpling 

 of the strata, deeply eroded, and then sunk again below sea level to receive 

 the second series of strata. The length of the [two periods of repose is 

 roughl}^ measured by the thickness of the two conformable series. The 

 length of the period of commotion is roughly measured by the amount of 

 erosion at the line of unconformity. 



Evidently, therefore, every case of unconformity marks a period of time 

 — often a long period — during which there was no record made in strata 

 and fossils at the observed place ; certain leaves — frequently very many 

 — are there missing from the Book of Time. Is it any wonder, then, that 

 skipping over these pages when we commence reading again we find the 



