Lituminoid Cement of the Caithness Flagstones. = 323 
which state it at once sinks to the bottom. If at the bottom 
of the lake there exists any decomposing vegetable matter, 
this brings about a redissolution of the ferric hydrate, owing 
to the abstraction of the extra molecule of oxygen by the 
carbon of the decomposing vegetable matter. This extra 
molecule of oxygen helps to form one or other of the humus 
compounds, which, having a powerful affinity for iron, quickly 
dissolve it, and carry it back again into solution, where it 
goes through the same changes as before. Solution and 
precipitation thus follow each other indefinitely. In those 
cases where decomposing organic matter at the bottom of the 
lake is small, or is absent entirely, then the balance is in 
favour of precipitation—the iron ore taking the form of the 
nearly indestructible ferric oxide, which becomes diffused 
through all the permeable strata at the bottom of the lake ; 
and the quantity of iron accumulating will be limited only 
by the two factors of the rate of supply and the duration of 
the conditions referred to. Where, on the other hand, decom- 
posing organic matter in any quantity is present in the lake, 
the iron can never be precipitated as ferric oxide, but will be 
diffused through the strata in the form of the ferric hydrate, 
or, under particular circumstances, as ferric carbonate. 
Reduction renders iron soluble and locomotive: oxygenation 
arrests locomotion, and fixes the iron on the spot in the form 
of the ferric oxide. | 
All this is in perfect accordance with the facts met with 
in the field. In all strata which are coloured red by ferric 
oxide at the time of their deposition, there is a conspicuous 
absence of vegetable matter in any form. Bituminoids, 
also, are absent in this case. But if, in connection with 
these Red Rocks (be their position in the geological scale 
what it may), there occurs a zone of strata in which the rocks 
are not coloured red by original ferric oxide, it is in that 
zone that organic remains may be expected to be met with. 
The grey beds of the Forfarshire Flags present us with a good 
illustration of this principle. 
It may be remarked here that the infiltration of ferric 
oxide from the lake bottom into the strata beneath may 
proceed to a considerable depth. In the case of the Carboni- 
VOL, XIII. Z 
