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Proceedings of the Koyal Society of Edinburgh. [Sess. 
to prove that bacteria or other organisms collect iron on their membranes, 
but quite another to show that rocks were built by this process. The 
works of Ehrenberg and of Senft have not justified this claim. In more 
modern times Molisch (4, i) has taken up this question. He records the 
presence of micro-organisms only in 3 out of 34 samples of ironstones 
examined by him ; and still later (4, ii), out of 27 samples, in only 1 case were 
there traces of micro-organisms. Thus, out of 61 samples it is in 4 cases 
only that he finds traces of micro-organisms. The organisms found by him 
were in all 4 cases iron-bacteria. He therefore came to the conclusion that 
in some cases ironstone may be formed from the debris of iron-bacteria, but 
that in the majority of ironstones iron-bacteria played no part. The in- 
formation which Molisch gives of the 4 samples in which iron-bacteria were 
found is not sufficiently complete to follow him with confidence. It gives 
one the impression that the iron-bacteria which were undoubtedly present 
in the stone were present in the deposit of ochre-coloured powder on the 
surface of the stone, and most probably of recent origin. There was 
nothing to indicate that the rock itself had been built up by the iron- 
bacteria in the same way that diatomaceous rocks have been built up by 
diatoms. It is impossible to speak with certainty, for we are told only the 
names of the countries in which these rocks were found, and nothing of 
their structure, or age, or any other points which seem necessary to carry 
conviction. The matter is one of great theoretical importance, and we 
require somewhat more elaborate and extensive information before we can 
speak with certainty on the question whether iron-bacteria were active 
agents in the formation of ironstones in any single instance. 
I have extended Molisch’s observations by examining the ironstones of 
Great Britain. The rocks examined were 48 in number, and were either 
ferruginous limestones or ironstones. They had all at some time or other 
been gathered in connection with ironstone borings. They included stones 
from the Primary rocks of Wales, from the Secondary rocks of the island 
of Skye (Scotland), from the Secondary rocks of the well-known Froding- 
ham Ironstones of Lincolnshire, and from the Secondary and Tertiary 
rocks in the neighbourhood of Hover and Folkestone. Iron-organisms, 
i.e. organisms which had deposits of ferric hydroxide on their outer 
surfaces, were found only in 4 among this number. The whole of 
the remainder, though showing organic remains in abundance, showed 
no traces of micro-organisms that had a chemistatic affinity for iron 
compounds. ' 
An iron-absorbing organism was found in each of these 4 slides in 
sufficient abundance to affect to some extent the percentage of iron in the 
