ART. 12, SIDERITE AND ASSOCIATED MINERALS—SHANNON. 13 
As shown by these analyses the material is ferrous carbonate with 
minor amounts of lime and magnesium carbonates and a very little 
manganese carbonate. The variation in composition from cavity to 
cavity is slight, and there is no essential difference in composition 
between the siderite of the first and second generations. 
In only one specimen were simple crystals of the siderite found. 
These were yellowish translucent unit or cleavage rhombohedrons 
showing only the form r (1011), as shown in sketch in figure 11 and 
in orthographic and clinographic projection in figure 12. These crys- 
tals, which averaged 2 mm. in diameter, occur scattered over the 
brown opal layer between nodules of ordinary second-generation 
sphaerosiderite. 
In one specimen only is there evidence of a third growth of sider- 
ite, although the crystals noted above may belong to a period of 
deposition later than that of the second-generation nodules. In the 
specimen mentioned a cavity contains a dozen spheres of ordinary 
second-generation sphaerosiderite averaging 6 mm. in diameter, which 
rest, as usual, upon the brown opal crust. Coating the exterior of 
these spheres and evidently a later growth is a crust, 0.75 mm. in 
average thickness, of crystalline siderite made up of innumerable 
closely crowded rhombohedral crystals one-fourth mm. in diameter. 
The crystal crust is loosely attached to the earlier nodules and can 
be easily scaled off. The smooth surfaces of the original nodules are 
coated with a layer or film of rusty limonite, which looks as though 
some oxidation bad taken place before the deposition of the crystal 
crust of the siderite. Scattered over the opal between the nodules 
are several typical saddle-shaped groups of small translucent brown 
rhombohedral siderite crystals. 
The form of these siderites is that of typical sphaerosiderite. 
While botryoidal forms are not rare in other carbonates of the same 
group, being characteristic especially of smithsonite, these forms, 
for smithsonite at least, grade imperceptibly into the crystalline- 
euhedral modifications. Such botryoidal, mammillary, and sphe- 
roidal forms have been called colloform by Rogers!’ because they 
are the forms assumed by colloids, which are devoid of any definite 
tendency to assume crystal form, and thus tend to form spheres. 
As has previously been pointed out, minerals form spherical bodies 
because they were deposited first as colloids and have retained the 
colloidal form after crystallization. As perhaps the most typical 
example of this process chalcedony may be mentioned. This mineral, 
though now crystalline, owes its form undoubtedly to its being origi- 
nally deposited as opal, and the rearrangement during crystallization 
has not been sufficient to destroy the original form. On the other 
hand, some minerals form spherical bodies when deposited in crystal- 
17 Austin F. Pogers. A review of the amorphous minerals. Journ. of Geol., vol, 25, 1917, p. 514. 
