Page:The Journal of geology (IA journalofgeology21894univers).pdf/453

coastal plain, except at two or three localities. The topography of all of these formations was greatly modified by erosion during intervening periods of high level of the land, but in general, the successive formations were planed nearly to base level before the succeeding deposits were laid down. This was the condition before the Lafayette epoch, and the seaward slope of the country was more gradual than at present, although the continent was high enough to allow the submerged continental shelf to be a sub-aërial plain. Then came the extensive subsidence and seaward tilting, which allowed the invasion of oceanic waters over the coastal plain, so as to permit of the deposition of the loams even upon the margin of the Piedmont plateau. This subsidence was unequal, least in the region of Cape Hatteras, greater along the South Carolina axis, again diminished in the Gulf region, and greatest along the Rio Grande. The author regards all of these Lafayette deposits as having accumulated at sea level from the land wash brought down by the rivers. Although devoid of marine life, so far as known, this seems the most rational explanation, although the physical characters are very different from those of the earlier Tertiary or Mesozoic deposits, which were laid down after submergence with less decided seaward tilting.

Mr. McGee regards the duration of the epoch of subsidence as short. The succeeding elevation, which carried the country from 100 to 1,000 feet above tide, he regards as much longer. This uplift was not uniform, probably only 100-300 feet at Cape Hatteras, and 1,000 feet at the mouth of the Mississippi, but in undulations such as characterized the previous subsidence; where the greatest depression had taken place, there the greatest elevation followed along the same axes. Moreover, it is apparent from the intensity of erosion that the elevation was greater along the Appalachian and Cumberland plateaus than along the coast, giving greater slope to the rivers than at present. This elevation was unquestionably of long duration and the erosion enormous, removing from the valleys a large proportion of the accumulations of the preceding epochs and cutting through them to depths of 150 feet and upward, and to widths of 10 and 20 miles, even 100 miles in the case of the Mississippi. This the author emphasizes, giving great prominence to the geomorphy from which the post-Lafayette elevation is deduced.

After this long-continued period of degradation, the continent subsided, but not so much as during the Lafayette days, and during