82 IRON AND MANGANESE ORE 
to be entirely composed of the organisms mentioned above 
and free from colloidal ferruginous material, it is evident 
that the organisms contained the iron. This conclusion 
was made still clearer by passing dilute hydrochloric acid 
through the residue consisting of the organisms, and then 
testing the solution with ammonia for iron. The resulting 
precipitate of hydrated ferric oxide showed that the organ- 
isms contained the iron. 
When early workers were first acquainted with “iron 
bacteria ’’ some of them contended that the deposition 
of ferric hydrate in.or upon ,the organisms was of a 
mechanical nature; but the views of Winogradsky and 
others, which regard the process as a physiological one, 
have been proved, or at least are fairly generally accepted. 
The abundance of micro-organisms in ferruginous 
material in bogs and streams is probably related to the 
known effect of iron on plant growth, which appears to be 
stimulated in proportion to the amount of iron present.* 
If observations in different parts of the world sub- 
stantiate it, the organic interpretation of the origin of 
the ore should be commendable to reason because the 
accumulations composing the ore would be attributable 
to the selective and cumulative functions with which 
organisms are recognised to be endowed. 
The writer wishes to acknowledge his obligation to 
Drs. T. H. Johnston and H. C. Richards, of the Queens- 
land University, for their generous assistance and useful 
criticisms. 
* Peirce, ‘‘ Plant Physiology, ” 2nd Edn., 1909, p. 101. 
