GEOLOGY OF BOTTOM OF SEAS—DE LAUNAY. 347 
ammonium, but which may also be in the state of alkaline phosphates 
or of phosphate of calcium, gradually impregnates the adjacent 
strata, especially those that are calcareous (polyps, corals, etc.), and 
sometimes even the clay, and becomes substituted for them, tending, 
through a correlative attraction exercised on fluorine, to take the 
composition of apatite, which is a crystallized fluo-phosphate of lime. 
The study of ferruginous sediments offers hke data for very inter- 
esting oceanographic observations on the interpretation of our iron 
ores. 
In the present marine deposits, independent of sublittoral oozes, 
we find iron either in the form of glauconite, or as red clay, each of 
these formations corresponding to a different origin and conditions. 
Glauconite of the present seas is a silicate of iron and potassium 
which chiefly characterizes some terrigenous deposits, particularly 
the muds and greensands. It is found along the continents, at a 
depth not exceeding 2,000 meters, but only where the terrigenous 
sedimentation is slow, without heavy fluvial supplies and where, con- 
sequently, the phenomena of solution and reprecipitation have time 
to act. It is evident that the iron of this mineral comes from the 
rocks and strata remolded by the sea, as its potash has for its origin 
the feldspars and potassium micas. For the alumino-alkaline sili- 
cates which chiefly characterize all our rocks is substituted a ferro- 
alkaline silicate, the alumina separating out from the other part as 
clay, to enter presently in the composition of the chlorites. This 
substitution is one of the first essential effects of the solvent reactions 
exerted by sea water, and we may at once note that lake water is 
powerless to produce a like chemical transformation. There is no 
glauconite in lake waters, probably because the iron in these waters 
is readily dissolved by the organic acids and immediately peroxi- 
dized by the excess of oxygen in them, being brought to precipita- 
tion under various forms, the most characteristic of which is bog 
iron, or limonite, instead of combining with the silica. Glauconite 
has a certain tendency to form small nodular grains not exceeding 
one-tenth of a millimeter. I have just remarked that it is frequently 
associated with phosphate, and like this has a tendency to develop 
by epigenesis, for example, from grains of feldspar. Like silica, the 
role of which is very similar, glauconite becomes solidified first on 
porous objects which have contained eliminated organic matter, such 
as the pores of foraminifera, or in the thin fissures of minerals. Then 
begin in the still submarine deposit renewed movements, some of. 
which, purely mechanical, accumulate the grains of glauconite at 
certain points by levigation such as has taken place in the tufas, 
and other movements, of chemical origin, result in the gradual ¢ con- 
centration of iron. 
