Page:Popular Science Monthly Volume 4.djvu/545

Rh determinate form. Next comes the history of the processes by which system is induced. There were hollows and valleys in those early times, most probably vast and deep, but not irregular. The constant fall of rain must originate brooks and streams, coursing their downward way toward the lowest levels. Animated with this descending impulse, they remove barriers at the outlets of lakes and pools, excavate gorges through ridges of impediment, and wear off numerous fragments from every projecting point. This eroded material would be urged forward by the current till the lowest possible level was reached, probably the bottom of an arm of the sea or bay, and remain there while the basin was filling up. Thus we should have a formation, composed of layers of the sand, clay, and limestone, originally a chemical precipitate, but now altered into sedimentary deposits. When the first accessible hollows had been filled up, a great interval of time had elapsed, and the external envelope of the earth would shrink, on account of its refrigeration, and fall upon the collapsed nucleus. Hence new valleys would be formed, and the streams would carry the detritus into them, and another set of strata lying upon the edges of the first formation would be deposited. This process has been going on uninterruptedly from that day to the present, and the face of the earth has been changed a hundred times. How long this process went on before the introduction of life it is impossible to say, for the oldest strata known to exist contain the remains of the Eozoic reef-building colonies, in the formations known as the Laurentian.

As some of the older Laurentian beds are composed of pebbles, it is obvious that earlier formations exist, from which the sedimentary material has been derived. Possibly we may be able ultimately to separate from the various systems of the age under consideration those characterized by the presence of the first existing plants—since in the order of Nature there must have been plants before animals. If we follow the analogy of the duration of the earlier periods, we may believe that this Eophytic age exceeded the Eozoic in length; and, furthermore, that the time before the introduction of life was far greater than what has lapsed subsequently. If the law admits of universal application, that the simpler the organism the longer it has lived, then we may perhaps claim that the earlier the period the greater has been its duration. The extent of work performed in these early ages has certainly far exceeded any thing yet known of the operations in the Zoic periods.

The series of changes prior to the introduction of life may therefore be registered as distinct ages, as well marked by special features and a natural order of succession as the periods defined by Paleontology. The minute details of the history are wanting, but, with such substantial bases of probability as have been set forth, human thought will construct theoretical systems that will command universal acceptance.