BY W. D, FRANCIS. 115 



that obstruct the River Congo Dr. Koenig has placed 

 in the British Museum, beside the syenites of the Congo, 

 the granites of Atures (Orinoco). . . . The black 

 crust is composed, according to the analysis of Mr. 

 Children, of the oxide of iron and manganese. . . Some 

 experiments made at Mexico, conjointly with Senor del Rio, 

 led me to think that the rocks of Atures which blacken the 

 paper in which they are \\Tapped, contain, beside oxide of 

 manganese, carbon and super- carburetted iron." 



Incrusting organisms may have caused these foreign 

 coatings. Evidence which is suggestive of organic origin 

 is afforded by Humboldt and 8enor del Rio's experiments 

 indicating the presence of carbon and super- carburetted 

 iron in the Orinoco incrustations. The environment 

 of the incrusted rocks of the Orinoco, as described by 

 Humboldt, resembles that of the Kin Kin examples in at 

 least two respects, namely, clear streams and abundant 

 vegetation. 



Acknowledgments . I am indebted to Mr. C. T. White, 

 F.L.S., Government Botanist, and Dr. J. Shirley, F.M.S., 

 for some suggestions : to Professor T. H. Johnston, 

 M.A., D.Sc. for kindly allowing me to use the photo- 

 micrographic apparatus of the Biology Department of 

 the University ; and to Mr. 0. W. Tiegs, B.Sc, Walter 

 and Eliza Hall Fellow in Biology, for his assistance »vith 

 the photo- micrographs. 



REFEREXCES. 

 1 Humboldt. " Personal Narrative," translated and edited by 

 Thomasina Ross, 1852, Vol. 11., pp. 243-246. 



2. L. C. Ball. " Queensland Government Mining Journal," Feb., 1918 



Vol. XIX., p. 57. 



3. De Bary. " Comparative Morphology and Biology of the Fungi 



Mycetozoa, and Bacteria, English translation, 1887, p. 408. 



4. De Tom. Sylloge Algarum, 1897, Vol. IV., p. 1714. 



5. H. J. Carter. " 8eeman"s Journal of Botany," 18G4, Vol. IL, p. 225, 



Plate XX 



6. Rabenhorst. " Flora Europaea — Algarum," 1868, section III.. 



p. 409, with figure 



7. Francis ^YoLLE. " Fresh-water Algae of the United States," 1887- 



text pp. 61, 62, Plate LXIX, figs. 17 and 18. 



8. Darwin. " A Naturalist's Voyage Round the World." 2nd Edn. 



pp. 12 and 13. under Bahia. 



