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SUPPOSED GLACIATION OF BRAZIL.
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of the Sugar Loaf at the entrace to the harbor of Rio, at the east base of the Corcovado, and about every such mountain in the vicinity of Rio de Janeiro. They[1] rest on the summits and margins of the high, sharp mountain peaks; on the top of the Sugar Loaf at the entrace of the Rio harbor, for example, there are several such boulders, one of which is thirty feet in diameter; the top of the Gavea, the flat-topped mountain southwest of Rio, has hundreds of boulders on its summits. Agassiz mentions such boulders on the edge of rock basins (Journey, 493). He "was at a loss how to explain how loose masses of rock, descending from the heights above should be caught in the edges of these basins, instead of rolling to the bottom." The fact is that the blocks referred to originated, not in the heights above, but just where they now lie, as is shown beyond question by occasional quartz veins passing from the boulders into the rocks upon which they rest.[2]

In some of the shallow parts of the Bay of Rio de Janeiro what were once small islands have had the residuary soils removed and great nests of such boulders project from the water.[3] On the island of Paquetá in the bay are some beautiful examples of such boulders lying in the water's edge. I am fortunately able to give an illustration showing the Paquetá boulders which may be taken as a type of those found in and about the Bay of Rio de Janeiro.

The second method by which these boulders have been

  1. ↑ Sometimes boulders accumulate on one side of a hill or peak and not on the opposite side. This is well illustrated in the case of the Sugar Loaf. On the side facing the ocean there are thousands of boulders, many of them of enormous size, while on the opposite side where there is less surf there are but few. The reason for this difference is that there is a large dike-like ledge of hard rock exposed on the seaward side of the peak. This ledge does not appear on the opposite side where the mass is softer and weathers away evenly without leaving good boulder-forming fragments about the base. The ledge referred to is shown in the accompanying illustration.
  2. ↑ In Shaler and Davis' Glaciers, plate XXII., is given an example of boulders of decomposition in Central India. Exactly similar cases are common in the granitic and gneissic areas of Brazil.
  3. ↑ See also Burmeister's Reise nach Brasilien, Berlin, 1852, 111, 112.
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