Page:Popular Science Monthly Volume 56.djvu/572

556 give rise to maximum erosion, and therefore abundant sediments of sandstone and clay, and comparative poverty of life and therefore of limestone. Chamberlin also gives reasons why the oceanic periods should be warm, humid, equable in temperature, and the atmosphere highly charged with CO2, and therefore highly favorable to abundant life, both vegetal and animal, while land periods would be drier and cooler, the atmosphere deficient in CO2, and therefore cold from that cause and in many ways unfavorable to abundant life.

These extremely interesting views, however, must be regarded as still on trial, as a provisional hypothesis to be sifted, confirmed, or rejected, or in any case modified, in the next century.

Lastly, it is interesting to note the ever-increasing part taken by American geologists in the advance of this science. There has been through the century a gradual movement of what might be called the center of gravity of geological research westward, until now, at its end, the most productive activity is here in America. This is not due to any superiority of American geologists, but to the superiority of their opportunities. Dana has well said that America is the type continent of the world. All geological problems are expressed here with a clearness and a simplicity not found elsewhere. We must add to this the comparative recency of geological study in this rich field. In Europe the simpler and broader problems are already worked out, and all that remain are difficult problems requiring much time. In America, on the contrary, not only are all problems expressed in simpler terms, but many great and broad problems are still awaiting solution. For these reasons the greatest activity in research, and the most rapid advance during the next century, will probably be here in America.



N many places in the extreme Southern States, especially in what is locally known as the "piney woods," one of the most notable features is the constantly recurring mounds of yellow sand which everywhere dot and, it must be confessed, disfigure the monotonous landscape. These piles of earth are usually nearly circular in form, fairly symmetrical in contour, from six inches to two feet in diameter, and, save where they have been beaten down by rain or winds or the trampling of cattle, about half as high as they are broad. Often these sand heaps are pretty evenly distributed, sometimes so thickly as to cover at least one fourth of the