Page:Popular Science Monthly Volume 59.djvu/261

Rh change be from warm to cold, cooling waters tighten their grasp on the precious gas that might offset the atmospheric depletion. If they be warmed beneath an air growing rich in carbon dioxide, they become generous of their hoard. The processes are, therefore, auxiliary and intensifying, not initiative.

For the initiative process, which may start the train of effects leading to atmospheric enrichment and a warm epoch, we must refer again to the periodic rest and unrest of the earth's forces.

When, through adjustment of the relations between continental masses and masses beneath the oceans, the internal stresses of the globe have been balanced, the average elevation of lands above sea level is a maximum. The highest rate of consumption of carbon dioxide by weathering may be assumed to follow after a brief but appreciable interval, and from that time forth to diminish. As heights waste and slopes sink low, they become mantled with the residual product of weathering, soil, and efficient contact of carbon dioxide with unaltered rock is limited. When the average height of land is become that of a low plain, carbonation is reduced to a very small part of its maximum activity, and the rate of consumption is slow. At some stage of this change the diminishing rate of depletion may equal and thereafter sink below the rate of supply. Thus the initial condition of a return of milder climate inheres in the transient nature of the cause of a cold epoch. The result might, however, be long delayed, but for the accelerating influence of auxiliary processes, of which, as already stated, life is believed to be the most potent.

It is a recorded fact of geologic history that periods of minimum continental elevation have been periods of extensive marine expansion. These conditions have been associated with remarkable development of marine faunas and with general mildness. Chamberlin was the first to point out a causal relation between these conditions and effects. He entertains the idea that at the climax of an epoch of crustal adjustment, the elevation of continents may be somewhat greater than that required by radial equilibrium. If so, they should in time exhibit a tendency to settle back. In so far as this period of readjustment of balance might suffice for deep general denudation, the subsiding lands would present low plain surfaces to the sea. These conditions would be most favorable for wide migration of the shore from the continental margins far inland, and would result in extensive areas of relatively shallow water. A fauna, which had existed on the narrow slope between the original position of the shore and an oceanic basin, would find its habitat immensely enlarged and favorably conditioned. Responding, it would develop varieties, species and genera until, as sometimes was the case, exuberance ran into unfitness and decadence.

The lowland aspect of continents would be favorable to other