THE OKIGIN OF BLACK COATINGS OF IRON ANO 

 MANGANESE OXIDES ON ROCKS. 



By W. D. Francis, Assistant Botanist, Queensland 

 Herbarium. 



{Read before the Royal Society of Queemland, June liOfh, 1920), 



(Plate I.) 



The coatings or incrustations dealt with particularly 

 in this paper are found on rocks in or near freshwater streams 

 in the Kin Kin district, which is situated about 100 miles 

 north of Brisbane, and about 10 miles from the coast. 

 Humboldt 1 describes and discusses coatings of apparently 

 similar chemical composition observed by himself and 

 others in the cataracts of the Orinoco, Nile and Congo. 

 These will be referred to in the latter part of the paper. 

 So far as I am aware, no explanation of the origin of the 

 coatings has been advanced except that offered by direct 

 chemical deposition upon the rock surfaces. 



Kin Kin district is an area of about 30 or 40 square 

 miles in extent originally consisting of dense rain forest 

 land, much of which has been felled and grassed during 

 the past 15 years. The soil of the greater part of the area, 

 especially in the east, is derived from schists and slates, 

 which, as stated by L. C. Ball 2, have been referred to the 

 Gympie Formation (Permo- Carboniferous). In the western 

 part are ranges strewn with boulders of a granitic rock. 

 The streams, which flow in an eastward or north- tastwa id 

 direction, contain clear water flowing over rocks of schist, 

 slate, quartz and grano-diorite (?). 



(1) Lichens. 

 The black coatings are divided into two kinds for the 

 purpose of this paper. The kind that is less frequent will 

 be discussed first. It is of a dull black colour, with a 



