4. After these periods of migrations and minglings there would be re-isolations in new localities, and the process of diversification would recommence and increase as long as the isolation continues.
The last of these critical periods of migrations and minglings and struggles for life among competing species was the Glacial epoch or ice age. Therefore the present geographical distribution of species was largely determined by the extensive migrations of that time.
II. Cosmopolitan and Provincial Faunas.—There are apparently in the history of the earth periods of widespread or cosmopolitan faunas, alternating with localized or provincial faunas. The cosmopolitan periods are usually times of prevalence of limestones or organic sediments, and the fossils are very abundant. The provincial periods are usually characterized by sandstones and shales or mechanical sediments, and are comparatively poor in fossils. Moreover, it is believed that the cosmopolitan limestone periods are oceanic periods—i. e., periods of wide oceans and lower and smaller continents and little erosive activity, while the sandstone periods, characterized by provincial faunas, are periods of higher and larger continents, and therefore of great erosion and abundant mechanical sedimentation.
Now, according to Chamberlin, these remarkable alternations are due to oscillations of the crust, in which the continents are alternately lifted and depressed. It must be remembered that abyssal faunas are almost unknown among fossils. This is the necessary result of substantial permanency of oceanic basins. The whole geological record is in shallow-water faunas. These shallow waters are along continental shore lines and in interior continental seas. According to Chamberlin again, during a period of continental depression all the flat continental margins are submerged, forming broad submarine platforms, and the lower interior portions of the continents are also submerged, forming wide and shallow interior seas. Under these conditions continental waste, and therefore sand and clay sediments, are reduced to a minimum. Life, animal and vegetal, abounds, and therefore much limestone is formed. The oceans are widely connected with one another, and therefore the faunas are widespread or cosmopolitan. During the period of elevation, on the contrary, the continents are extended to the margin of the deep oceanic basins, the broad, shallow submarine platforms are abolished, the interior seas are also abolished, the shallow-water areas are reduced to isolated bays, and their faunas are peculiar or provincial. Also, elevated and enlarged continents 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.
"SALAMANDERS" AND "SALAMANDER" CATS.
By NORMAN ROBINSON.
In 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 soil surface. If you ask a native the cause of this singular phenomenon, which you will perhaps at first be disposed to consider a kind of arenaceous eruption which has somehow broken out on the face of Nature, your informant will sententiously reply, "Salamanders!"