SUPPOSED GLACIATION OF BRAZIL. 763 



of the Sugar Loaf at the entrance 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 entrance of the Rio harbor, for example, there 

 are several such boulders, one of which is thirty feet in diame- 

 ter ; 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, descend- 

 ing 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 occa- 

 sional 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 Paqueta 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 Paqueta 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, in, 112. 



