Note 102 — Chapter XIII · PHYSICAL HISTORY OF THE AMAZONS.

As I have stated in the beginning, I am satisfied that the unstratified clay deposit of Rio and its vicinity is genuine glacial drift, resulting from the grinding of the loose materials interposed between the glacier and the solid rock in place, and retaining to this day the position in which it was left by the ice. Like all such accumulations, it is totally free from stratification. If this be so, it is evident, on comparing the two formations, that the ochraceous sandy clay of the valley of the Amazons has been deposited under different circumstances; that, while it owes its resemblance to the Rio drift to the fact that its materials were originally ground by glaciers in the upper part of the valley, these materials have subsequently been spread throughout the whole basin and actually deposited under the agency of water. A survey of the more southern provinces of Brazil, extending to the temperate zone, where the combined effects of a tropical sun and of tropical rains must naturally be wanting, will, I trust, remove all the difficulties still attending this explanation. The glacial phenomena, with all their characteristic features, are already known to cover the southernmost parts of South America. The intervening range, between 22° and 36° of south latitude, cannot fail to exhibit the transition from the drift of the cold and temperate zone to the formations of a kindred character described above from the tropical zone. The knowledge of these deposits will definitely settle the question; and either prove the correctness of my generalizations or show their absurdity. I feel no anxiety as to the result. I only long for a speedy removal of all doubts.