390 TJie American Geologist. December, i89i< 
Lacroix, M. Cayeux is now able" to add to the simple announcement 
of double refraction of glauconite made by him in 1892, the essential 
characters of a crystalline substance, viz: it is biaxial (sometimes ap- 
parently uniaxial), the angle of the optic axes being from 30 to 40 
degrees. The value of ;/g— /Zp is about 0.020, and from analogy 
with the micas and chlorites it is supposed to be monoclinic. Glau- 
conite has also been found, though very rarely, to have a radiated- 
fibrous or concretionary structure, giving a black cross in thin sec- 
tion. 
In the discussion of the phenomena of alteration of glauconite, 
the author bears directly on the theory of Spurr of the origin of the 
great deposits of iron ore of the Taconic in Minnesota. He says; 
"The products of decomposition of glauconite are hydrates of the 
oxide of iron and pyrite." Most frequently alteration begins to ap- 
pear at the periphery and extends progressively and regularly to the 
interior. It also reaches the center of the grains by way of fissures 
which traverse them in all directions. The products of decomposi- 
tion appear not only throughout the areas that are altered but they 
migrate sometimes and color the surrounding parts and encrust sur- 
rounding minerals. Alteration may make its first appearance at the 
center of the grains as in the Meule de Thivencelles, for example, 
where it affects a great number of grains; grains are there 
found transformed into hydrated iron oxide excepting only a slight 
superficial zone which remains intact." 
Matthew has also found a notable amount of oxide of iron in the 
Cambrian of New Brunswick wherever foraminiferal remains, and 
hence glauconite, occur, the two bemg reasonably taken to be com- 
plementary. 
According to Dr. Cayeux: (i) Glauconite has been formed in 
other places than those in which its grains now exist, and has shared 
in the processes of mechanical transport in the same manner as grains 
of sand. (2) It contains inclusions of other minerals, such as magne- 
tite and quartz. (3) Forms incrustations on crystals of orthoclase, 
microcUne, etc., penetrating along the cleavages even to the center 
of the grains. (4) Serves as a pigment in the same manner as 
limonite. (s) Takes the form globulaire, like nascent quartz crystals 
described by Fouque, and by combination of these globules grows 
into larger and larger groups and masses. (6) Its grains take com- 
plete outward form but do not sometimes fill up the interior, only a 
skeleton of glauconite occupying the space. (7) Continues to increase 
after the deposit of the rock which contains it. (8) Forms at different 
dates, cotemporary with the consolidation of the rock and posterior 
to it. (8) Sometimes replaces calcite, taking the form of calcite rhom- 
bohedrons. (9) Is usually, but not always, dependent on organic sub- 
stances for its genesis. n. h. w. 
