Page:The Journal of geology (IA journalofgeology11893univers).pdf/342

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 invariably 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 residuary 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 impenetrable jungles that cover the whole country and so effectually limit the range of one's observation. Dikes such as these last mentioned are not uncommon in the mountains of Rio de Janeiro. Indeed what have generally been regarded as the very best evidences of Brazilian glaciation,^[1] some of the boulders near the English 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