Vol. 62. .] CA.TAKACTS OF THE KIVEB MADEIRA, ETC. 121 



disturbed conditions, so that a normal granitic structure would be 

 developed in the minerals of the second stage. 1 



The haplite-dykes belong, no doubt, to the second stage of crystal- 

 lization, although they are somewhat posterior in time ; but most of 

 the other dyke-rocks belong, I believe, to a much later period. 



(6) The Black Coating of the Rocks of the Cataracts. 



On the rocks of the island below Santo Antonio the black deposit 

 referred to on p. 104 was well seen. A rough analysis, by Mr. Gr. S. 

 Blake, of a little of this material scraped from the surface of specimen 

 M 14 gave the following result : — 



Insoluble in acids 

 Soluble in acids . . . 



fSiO, 6-6 



t Residue (probably A1 3 ) 9'3 



j'Mn0 2 ". 54'1 



J Fe 2 3 6-5 



|CaO 1-3 



(MgO 00 



Undetermined components, in all probability mainly water 



and organic matter 22*2 



Total 1000 



The state of oxidation of the manganese was not determined, but 

 the material resembles psilomeiane (hydrated manganese-dioxide), 

 and it is in all probability a variety of that mineral. The presence 

 of silica and alumina is no doubt due to part of the surface of the 

 rock having been removed with the scrapings. 



Similar black deposits have been described as occurring on the 

 rocks of the Orinoco, the Upper Paraguay, the Nile, the Niger, and 

 the Congo, especially in the neighbourhood of cataracts. 2 



Apparently manganese and iron-oxides are more soluble in river- 

 water in the tropics than in colder countries. It is not clear that 

 higher temperature of the water is alone sufficient. Probably there 

 are organic compounds dissolved in the water, and the iron and 

 manganese either occur as salts of organic acids or as carbonates, 



1 Similar inclusions in felspar of quartz with rounded idiomorphic boundaries 

 occur in the granulites of Ceylon ; see Dr. Ernst Weinschenk, 'Zur Kenntniss 

 der Graphitlagerstatten ' Abhancll. der Matb.-phys. Classe der k. Bayer. Akad. 

 der Wissenscb. vol. xxi (1902) p. 298 & pi. vi, figs. 1-2 ; also ' Gruudziige der 

 Gesteinslehre ' pt. ii (1905) fig. 20, p. 51. For inclusions of quartz in horn- 

 blende, as in M 1 a, p. 97, see pi. v, fig. 4 of the former paper, and fig. 19, 

 p. 50 of the latter. 



2 See a paper by MM. Lortet & Hugounenq, Comptes Rendus Acad. Sci. 

 Paris, vol. cxxxiv (1902) p. 1091 (reviewed in the ' Geographical Journal' vol. xx. 

 1902, p. 655) ; ' The Geology of Matto Grosso ' Quart. Journ. Geol. Socl 

 vol. 1 (1894) p. 98 ; also Dr. W. Kcert, ' Geologisch-agronomischeUntersuchung 

 der Umgegend von Amani in Ostusambara,' Berichte iiber Land- & Forst- 

 wirthschaft in Deutsch-Ostafrika : vol. ii (1904) pp. 152 & 163. 



Q. J. G. S. No. 245. k 



