1914-15.] Fossil Micro-organisms. 121 
3. A third consideration is that an examination of the micro-oro^anisras 
that were found in other highly ferruginous rocks showed that in no tase 
was there any indication that they had accumulated iron on their mem- 
branes to a greater degree than any of the other organisms in these rocks. 
In the case of Phycomycites Frodinghamii the accumulation of iron on 
its membranes is an imposing feature of its appearance. Inasmuch as 
this is not the case either with the other organisms in the same rocks as 
those in which Phycomycites Frodinghamii were found, or in the case of 
other micro-organisms in other highly ferruginous rocks, we must assume 
that it is in the highest degree probable that the organism we are discussing 
possessed some special attraction for iron-compounds. 
In view of these points I consider that the mould in question was an 
iron-organism in the sense that it had a chemiotactic affinity for iron- 
compounds, as have at the present day the iron-bacteria, some algae, and 
some protozoa. 
Geological Considerations. 
In the geological explanations of the origin of the high content of iron 
in the Frodino^ham Ironstone, all agree that the iron has filtered into this bed 
subsequent to the formation of the stone, either from the rocks which are 
above or from those that are below. So far as the oolites are concerned 
there seems no other explanation than that the lime contained in them 
has been gradually replaced by a ferruginous salt. In the Frodingham 
Ironstone the oolites with which it abounds are reddish yellow in colour, 
and it is evident that the iron has penetrated throughout the whole mass 
of the oolite and is not merely a superficial coating. The considerations 
stated in the last section indicate, however, that the high content of iron in 
these stones cannot be completely accounted for by a theory of replace- 
ment. In order to account for all the facts of the case, we must assume 
that a start at any rate was given to the accumulation of iron by the 
chemiotactic affinities of the organism under discussion for iron-compounds. 
This probably took place when this plant flourished in highly ferruginous 
waters on and in the remains of animals which after death had become 
engulfed in such waters. The organic remains would thus become com- 
paratively highly ferruginous before those processes were initiated which 
later resulted in their fossilisation. So far, therefore, as the Frodingham 
Ironstone is concerned, quite an appreciable percentage of its iron must 
have been derived in the manner mentioned above. I propose dealing 
below with the questions as to the extent that micro-organisms have 
contributed to the formation of ironstones. 
