Page:The Journal of geology (IA journalofgeology11893univers).pdf/346

This page has been proofread, but needs to be validated.
768
THE JOURNAL OF GEOLOGY.

explanation of this water-worn material seems to be that the Tertiary period was closed by a depression along the present coast, which carried the beach line far inland, or that it was already there. Then followed a gradual emergence,[1] during which the whole area now covered by this widely distributed water-worn material was passed gradually through the condition of a beach, upon which the then loose, angular, surface rocks of the country were rounded and worn into the boulders, cobbles and pebbles which we now find scattered over this region. While the surf was beating upon and wearing the hard crystalline and metamorphic rocks of the interior it was unable to produce any very marked effect upon the topography of the country, but when, in the course of the land's emergence, the soft, sandy and clayey beds of the Tertiary were brought up within its reach, the work of land sculpture it was able to do was enormously increased. During the emergence of these Tertiary beds they were deeply eroded, and the mud which originally made part of them was washed seaward, and the coarser materials were concentrated upon the slowly receding beach. In some places these accumulations assume unusual proportions, as if they had been brought together by the gradual beating of waves along a beach, or had been reconcentrated by later streams."

GLACIAL TOPOGRAPHY.

Agassiz considered that the undulating outlines of the topography about Rio de Janeiro were attributable to glacial action,[2] though he recognizes the fact that nothing of glaciation was to be learned from their appearance.[3] A careful study of those features, made with the suggestion in mind, shows that the rounded hillsides have no uniformity in their arrangement, that is, what would be stoss sides, judging from the topographic forms, face now in one direction, and now in another, and that the outlines are simply those produced by ordinary decomposition and erosion, though much influenced by structural features. Hartt's opinion, as originally expressed in his book (p. 33), was that the forms of the hills were "due primarily to subaërial denudation."

THE ABSENCE OF STRIÆ.

A bit of negative evidence of great importance against the glacial hypothesis is the fact that nowhere has there been found

  1. ↑ See also Pissis in Comptes Rendus de l' Acad. des Sci., 1842, XIV., 1046.
  2. ↑ Geological Sketches, II., 157.Bul. de la Soc. Géol. de France, 1867-8, XXV., 687.
  3. ↑ Journey, pp. 69-70.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.