540 Critical Periods in the History of the Earth. [ September, 
ON CRITICAL PERIODS IN THE HISTORY OF THE 
EARTH, AND THEIR RELATION TO EVOLUTION; 
ON THE QUARTERNARY AS SUCH A PERIOD. 
BY JOSEPH LECONTE. 
— the series of rocks representing the history of the earth there 
occur at different horizons: wnconformities. In most cases 
these are not found at the same horizon in different 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 
unconformity. 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 at- 
tended with less sweeping change of organic forms, of the sub- 
divisions of time. 
The earlier geologists, under the influence of the then domi- 
nant 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 de- 
stroyed 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 subse- 
quent period of tranquillity. Now, however, we believe that no 
such instantaneous general exterminations and re-creations ever 
occurred. We know that unconformity simply indicates eroded 
land-surface, and therefore 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 denote two 
periods of comparative quiet, during which the observed place 
was sea-bottom, receiving sediment steadily, separated by a 
period of oscillation producing increase and decrease of la 
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 roughly measured by 
the thickness of the two conformable series. The length of yri 
period of commotion is roughly measured by the amount of ero- 
sion at the line of unconformity. 
1 Read before the National Academy of Sciences, April 18, 1877. ( Reprinted from 
the American Journal of Science and Arts, for August, 1877.) 
