348 The American Geologist. SURE ee 
writer quotes the first to recall that silicate of iron of chloritic 
appearance and habit, even when in eruptive rocks, is called 
glauconite. 
Concerning the origin of glauconite grains in sedimentary 
rocks, I translate from Zirkel.* 
“The microscopic investigations of Ehrenberg have shown that 
many glauconite grains are the casts of foraminiferal shells, which 
were filled with the glauconite substance and later dissolved. Reuss, 
while he could plainly recognize many of the glauconite grains investi- 
gated by him as incrustations or fillings of foraminiferal shells, or 
could conclude them to be the broken up fragments of such shell- 
fillings, nevertheless expressed himself against the universal applica- 
tion of Ehrenberg’s observation, and considered the great majority 
of the grains as concretions which had grown from the center out- 
wards. Also Gtimbel says, ‘While it is clear that glauconite forms 
in the cavities of foraminifera or other marine animals, such as small 
gasteropods, pteropods, serpulz, and ostracods, and while it is sure 
that many of the grains which are now mixed with the sand and are 
without an organic shell covering, owe their origin to the breaking 
up of dissolved shells full of glauconite substance, yet there are many 
other glauconite grains, which cannot be referred. from their form 
or their size, to such an origin.’ He believes these have formed as 
entoolites. According to this, a thin film of glauconite is deposited 
on the surface of gas-bubbles, and inside of these the further filling 
is accomplished by intussusception. He supposes this formation of 
glauconite to always take place near the shores or in inconsiderable 
depths of the sea, But the glauconite grains, which occur in the green- 
ish sands and bluish muds of the present deep-sea deposits, are part- 
ly quite evident casts of foraminifera, and in part have a rounded 
form with often warty surface; since there is often the appearance 
that the deposition of the glauconite has burst the foraminiferal shell, 
the supposition follows that the rounded grains are also casts, which 
continue to grow after the dissolution of the shell (Murray and 
Renard ).” 
From this it will be seen that although glauconite (for 
example, that in the igneous rocks) may form without the 
assistance of organic matter, yet in the sedimentary rocks 
this agency is most active in its formation. Where the or- 
ganic material is most abundant, as within the tiny shells 
of foraminifera, etc., the building of glauconite is most active, 
but it is certain that these glauconite grains grow up concre- 
tionary action often beyond the limits of the shell. It is also 
probable that small particles of organic matter, scattered 
through the rock, may each begin the precipitation of glaucon- 
ite, which may grow to a grain of considerable size. 
* Lehrbuch der Petrographie, 2d ed., vol. iii, p. 728. 
ee 
