Note 46 — ON STEPPES AND DESERTS.

p. 19—“Huge masses of leaden-coloured granite.”

In the Orinoco, and more especially at the cataracts of Maypures and Atures (not in the Black River or Rio Negro), all blocks of granite, even pieces of white quartz, wherever they come in contact with the water, acquire a grayish black coating, which does not penetrate beyond 0·01 of a line into the interior of the rock. The traveller might almost suppose that he was looking at basalt, or fossils coloured with graphite. Indeed, the crust does actually appear to contain manganese and carbon. I say “appears” to do so, because the phenomenon has not yet been thoroughly investigated. Something perfectly analogous to this was observed by Rozier in the syenitic rocks of the Nile (near Syene and Philæ); by the unfortunate Captain Tuckey on the rocky banks of the Zaire; and by Sir Robert Schomburgk at Berbice.^[HF] On the Orinoco these leaden-coloured rocks are supposed when wet to give forth noxious exhalations, and their vicinity is believed to be conducive to the generation of fevers.^[HG] It is also remarkable that the South American rivers generally, which have black waters (aguas negras), or waters of a coffee brown or wine yellow tint, do not darken the granite rocks; that is to say, they do not act upon the stone in such a manner as to form from its constituent parts a black or leaden-coloured crust.