On Critical Periods in the History of the Earth, and Their Relation to Evolution; On the Quarternary as Such a Period

Abstract

JN the series of rocks representing the history of the earth there occur at different horizons unconformities. 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 attended with less sweeping change of organic forms, of the subdivisions of time.The earlier geologists, under the influence of the then domninant 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 subsequent 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 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 roughly 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.

Catastrophism and uniformitarianism: logical roots and current relevance in geology

“…The American geologist Joseph LeConte (1877LeConte ( , 1895 recognized that the source of Lyell's methodology was physics, not geology. LeConte (1895, p. 315) wrote:…”