1914-15.] 
Fossil Micro-organisms. 
129 
Have Micro-organisms contributed to the Building 
OF Ironstones ? 
As far back as 1838 Ehrenberg (1) emphasised the importance of 
“ Infusorien ” as factors in the formation of certain strata. He stated that 
for the formation of ironstone the activity of certain organisms of the class 
of the diatom Gallionella ferruginea was indispensable. (Ehrenberg re- 
garded diatoms as members of the animal kingdom.) His observations 
were based on the examination of samples of ironstone from the neighbour- 
hood of Berlin, from the Ural Mountains, and from New York. It is 
evident that Ehrenberg had under examination organically formed siliceous 
rocks, the silica of which had been derived from diatoms, and which had at 
the same time a fairly large percentage of iron. It is now impossible to 
say what organism Ehrenberg had under observation, but it is certain that 
it was not what is now known as Gallionella ferruginea, which is one of 
the iron bacteria and not one of the group of diatoms. One point, however, 
is of interest, viz. that diatoms can and do collect ferric hydroxide on their 
membranes whenever they find acceptable conditions of life in ferruginous 
waters. It is more probable that Ehrenberg’s ironstones were siliceous 
ferruginous rocks in which the diatoms had supplied only the siliceous 
portion, and that the ferruginous constituent had independently made its 
appearance in the rock. No subsequent ironstone has been discovered in 
which the diatom was responsible for the iron constituent of the rock. In 
the same way Senft (8) interested himself in the origin of ironstones, and 
emphasised the importance of microscopic algm as rock -building agents. 
He also stated that living algae were busy in the formation of ironstone, 
particularly the Oscillatorieae. There are no known living algae, except the 
diatoms, which collect iron on their membranes in any appreciable quantity, 
and I agree with Molisch in considering that Senft probably mistook for 
Oscillatoria some of the members of the iron-bacteria. I would suggest 
that Senft was observing iron-covered threads of Leptothrix ochracea, 
which might under certain circumstances be easily mistaken for Oscilla- 
toria threads. It is evident that in the case of certain rocks the remains 
of algm have been instrumental in the formation of rocks. Thus, to 
mention only two, the work of Rothpletz (7) has shown the activity, in this 
respect, of the calcareous red seaweeds, and Nicholson and Etheridge (5) 
have shown the activities of the curious alga, Girvanella, in the formation 
of the Ordovician Limestone of Ayrshire. But as yet we have not a single 
indisputable case that ironstones were formed in this way. It is one thing 
VOL. XXXV. 9 
