Note 46 — STEPPES AND DESERTS.

p. 24.—“Massive leaden-coloured granite rocks.”

In the Orinoco, and more especially at the Cataracts of Maypures and Atures, all blocks of granite, and even white pieces of quartz, whenever they are touched by the water of the river, acquire a greyish-black coating which scarcely penetrates a hundredth of a line below the surface of the rock. The appearance produced is that of basalt, or fossils coloured with graphite. The crust appears to contain manganese and carbon; I say appears, for the phenomenon has not yet been thoroughly examined. Something similar was remarked by Rozier on the syenite rocks of the Nile, near Syene and Philæ; by the unfortunate Captain Tuckey on the rocky banks of the Congo; and by Sir Robert Schomburgh on the Berbice. (Reisen in Guiana und am Orinoko, S. 212.) On the Orinoco these leaden-coloured rocks are considered to give out pernicious exhalations when wet; and their proximity is believed to produce fevers. (Rel. hist. T. ii. p. 299–304.) In the Rio Negro, and generally in the South American rivers which have “black waters,” “aguas negras,” or waters of a coffee-brown or yellow tint, no such effects take place. No black colour is imparted to the granite rocks by the waters; that is to say, they do not act upon the stone so as to form from its constituent particles a black or leaden-coloured crust.