546 deport— 1878. 



There are also other deposits of iron ores, far more important, and which cover 

 immense tracts of land, especially in the tertiary formations ; many of them are 

 remarkable for their purity, and are free from debris of former rocks. 



Under what influence were some of these so-called limonites, or brown hema- 

 tites, formed? This problem can, I believe, be more or less solved by the 

 observation of a phenomenon going on in some of the large iron tanks holding 

 Yartry water. 



We have in the College of Science, Dublin, a large iron tank on the roof of the 

 house, into which a constant supply of water flows freely. After a certain lapse of 

 time, the bottom of the tank and also the sides are covered with sponge-like concre- 

 tions, assuming the various shapes of the well-known concretions of lime or iron 

 ore found in various geological formations. The above sponge-like deposits are, 

 in our case, nothing else but vegetable microscopic organisms— billions, I may say, 

 of thread moulds, Penicillia. 



These organisms accumulate after many months to such an extent as to form an 

 immense sponge intercepting the flow of water. Nevertheless, in _ the case of the 

 iron tank, the water remains pure and pleasant to drink, and is without any taste 

 of iron. 



These organisms have, I may say, assimilated the iron (as hydrated sesqui- 

 oxide) and made of it a part of their constitution. 



During the process of life they have consumed the carbonic acid, and they 

 leave as a last result, when dried in a water-bath, a pure ore of iron, containing 

 a mere skeleton of carbon, and having a formula corresponding to that of brown 

 hematite. 



