764 THE JOURNAL OF GEOLOGY. 



formed is quite similar to the first, but instead of being cores of 

 granite or gneiss, they have been derived by the same process of 

 exfoliation and decomposition from the angular -blocks into 

 which the dikes of diorite, diabase, or other dark colored rocks 

 break up. Their color marks them as quite different from the 

 surrounding granites, and the dikes themselves are almost invaria- 

 bly concealed. Moreover, these dikes not infrequently contain 

 inclusions of still different rocks and we thus occasionally have 

 boulders of various kinds of rocks mingled together. The resi- 

 duary clays derived from the decomposition of these dikes are 

 somewhat different in color from those yielded by the granites, 

 so that when "creep" or land-slides add their confusion to the 

 original relations of the rocks, the resemblance to true glacial 

 boulder-clays is pretty strong. The chance of discovering the 

 source of these boulders is further decreased by the depth to 

 which the mass of the rock has decayed, and by the impenetra- 

 ble jungles that cover the whole country and so effectually limit 

 the range of one's observation. Dikes such as these last men- 

 tioned are not uncommon in the mountains of Rio de Janeiro. 

 Indeed what have generally been regarded as the very best evi- 

 dences of Brazilian glaciation, 1 some of the boulders near the Eng- 

 lish hotel in Tijuca, fall under this head, though some of them 

 are of gneiss. The fact is that the great mountain masses about 

 Rio are of granite or gneiss, while some of the boulders come 

 from dikes or other dark-colored rock high on their sides, dikes 

 which were not visited by Agassiz or Hartt. 2 There is a good 

 example of a dike breaking up in boulders at the gap through 

 which the road passes from the Jardim Botanico to the Gavea 

 near the City of Rio. At this place the ground is covered to a 



1 A Journey to Brazil, 86 et seq. ; Agassiz : Geological Sketches, Boston, 1885, 

 II., 155 et seq. Hartt's Geol. and Phys. Geog. of Brazil, 24-30. 



2 Darwin mentions boulders and dikes seen at Rio de Janeiro, (Geological Obser- 

 vations, pt. II., ch. XIII., 425; also Trans. Geol. Soc. London, 2d Series, 1842, VI., 427, 

 note). Professor O. A. Derby sent Rosenbusch specimens of diabase from twelve 

 dikes in the neighborhood of Rio de Janeiro, varying from twenty centimetres to sev- 

 eral metres in thickness. See Dr. E. O. "Hovey's descriptions of these rocks in 

 Tschermak's Min. u. Petrog. Mittheilungen, 1893, XIII. , 211-218. 



