BY W. 1>. FRANCIS. Ill 



somewhat granular surface, is fairly uniform in thickness, 

 varying from .085 to .136 mm., has only been observed 

 on the schists and slates, and can be removed in small 

 pieces with a penknife. The small pieces thus removed 

 are quite opaque when examined with the microscope, 

 and appear structureless. When fused with sodium 

 carbonate, thej^ produced the green colour reaction 

 indicating the presence of manganese. In hydrochloric 

 acid they partly dissolved, forming a pale brown solution 

 and a residual ]) leached or whitish matrix. On the addition 

 of ammonium hydroxide to the pale brown solution an 

 abundant precipitate of ferric hydrate indicated the presence 

 of iron in considerable quantities. Microscopic examina- 

 tion of the residual matrix showed that it consisted of fine 

 filaments and small round or subangular cells, evidently 

 the remains of the thallus of a lichen. Crustose lichens 

 are ver}^ abundant on the schists and slates of the dense 

 rain forests of the district, and it appears quite clear that 

 the iron and manganese compounds are deposited in, or 

 partly replace, the substance of the lichen thallus to form 

 the black coating. 



The incorporation of iron compounds in the substance 

 of the thalli of incrusting lichens is not unrecorded in 

 scientific work. De Bary^ states : — " Another series of 

 infiltrations and imbeddings is composed of inorganic 

 matter. First and foremost is the rust colour not 

 unfrequentl}^ assumed by individuals (' formae oxydatae ') 

 of many crustaceous lichens which are typically of another 

 colour ; it has often been stated and has now been proved 

 by Gumbel that this colour is due to the infiltration of a 

 salt of iron, perhaps of a vegetable acid." 



(2) Algae. 

 The other kind of black coating is more difficult to 

 investigate. It is much thinner, varying from about 

 3^ to 41,M in thickness, and is more firmlj' adherent to the 

 rock surface. It is very abundant on all kinds of rocks 

 in the streams, both in cleared and uncleared areas. 

 Hydrochloric acid was applied to it in order to assist in 

 the removal of minute pieces. It gave the same reaction 

 for manganese as the lichen- formed coating and dissolved 



