114 ROCK INCRUSTATIONS. 



thallus of the alga during partial decomposition, unless 

 its whole substance, including its very high percentage of 

 water, were replaced b}' iron and manganese compounds. 

 (See Figs. 1 and 4, Plate I.) 



Six black-coated rocks, each from different parts of 

 two streams, were chosen for microscopical investigation. 

 The small pieces of the coating removed bj' scraping were 

 opaque and apparently- structureless. Samples were then 

 carefuU}" treated with hj^drochloric acid in order to render 

 them transparent, when it was found that three out of 

 the six examples definitelj' showed the cellular structure 

 of the thallus of the alga. 



Review. 



While these investigations do not absolutely prove 

 the second kind of black coating to originate from the 

 incrusting alga, thej^ indicate, at least, that such a conclusion 

 is highly probable. Absolute proof would consist of the 

 production of the black coating bj* experimental processes 

 in the streams. Some experiments of that kind were 

 begun, but the rocks were lost trace of through the action 

 of floods. 



The black coatings described by Humboldt' from the 

 Orinoco, Nile, and Congo, and the brown ones observed 

 by Darwin^ at Bahia, in Brazil, are only known to me 

 from the authors' descriptions. The following extracts 

 from Humboldt's^ work summarise his remarks : — 



" xA.mong the cataracts and wherever the Orinoco, 

 between the missions of Carichana and of Santa Barbara, 

 periodically washes the rocks, they become smooth, black 

 and as if coated with plumbago. The colouring matter 

 does not penetrate the stone which is coarse grained granite. 

 The black crust is 0.3 of a line in thickness. 

 On ])reaking the stone with a hammer, the inside is found 

 to be white. . . . M. Roziere, who had travelled over 

 the valley of |Egyj)t, the coasts of the Red Sea and Mt. 

 Sinai, pointed out to me that the primitive rocks of the 

 little catarcts of Sj-ene display, like the rocks of the 

 Orinoco, a glossj^ surface, of a blackish grey or almost 

 leaden colour. ... . The English naturalists were 



struck with the same appearance in the rapids and shoals 



