ANNOTATIONS AND ADDITIONS. 189 



The appearance produced is that of basalt, or fossils co- 

 loured with graphite. The crust appears to contain man- 

 ganese 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 Philse ; by the unfortunate Captain Tuckey on 

 the rocky banks of the Congo ; and by Sir Robert Schom- 

 burgh 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 con- 

 stituent particles a black or leaden-coloured crust. 



( 47 ) p. 24. " The rain-announcing howlings of the 

 bearded apes." 



The melancholy howlings of the small apes, Simia seni- 

 culus, Simia beelzebub, &c., are heard some hours before 

 the rain commences : it is as if the tempest were heard 

 raging at a distance. The intensity of the noise produced 

 by such small animals can only be explained by their 

 number ; seventy or eighty being often lodged in a single 

 tree. On the organs of voice of these animals, see my ana- 

 tomical treatise in the first chapter of my Recueil d'Obser- 

 vations de Zoologie, vol. i. p. 18. 



