11) PROCEEDINGS OF THE NATIONAL MUSEUM. VoL. 62. 
The second generation siderite, as contrasted with that of the first 
generation, occurs much more abundantly in its specimens and as 
much larger masses, the individual spheroids reaching an extreme 
observed diameter of 4cm. Itis much more translucent than the 
early variety and has a darker olive-green color. This siderite of 
the later generation is always, so far as seen, deposited in cavities in 
the rock which is dark colored by reason of having undergone opal- 
ization, and the siderite in the cavities is practically always under- 
lain by a thin film of the ferric opal. 
A cavity may contain only a single globe of siderite, or it may be 
almost completely filled with crowded units (pl. 2). Not infrequently 
the spherulites have grown in linear arrangement and by their union 
have produced peculiar caterpillarlike forms, as shown in plate 2, 
lower. The surfaces of these dark-colored second-generation nodules 
of siderite are usually velvety in appearance or in some cases remark- 
ably and beautifully iridescent. Chance fractures show the nodules 
to have a radiate fibrous internal structure, although this is masked 
in most cases by the perfect rhombohedral cleavage of the mineral, 
yielding curved cleavage surfaces. Cross sections of the spheres when 
examined in polarized light show a homogeneous radiate fibrous 
structure in which individual fibers are so minute as to be indistin- 
guishable. Between crossed nicols the nodules do not extinguish, 
but yield the dark extinction cross characteristic of very finely fibrous 
spherules. There is no indication of concentric structure indicating 
periodic growth or interrupted deposition of the siderite, nor are there 
any inclusions in the section, although one sample analyzed was found 
to include a flake of hyalite, though its surfaces were entirely cleav- 
ages. The optical properties of a nodule of second-generation siderite 
were determined by Larsen, who found the indices of refraction to be 
w=1.851 and e=1.612. 
Four analyses of the Spokane siderite have been made with the 
results tabulated below: 
Analyses of sphaerosiderite from Spokane. 
i 2 3 4 
FaG@Oyei: svewle-aqsiagn af grads. tleead - 93.49 | 91.96 | 92.41) 93.35 
Mn CO gece oer ghee oe ats ol aae URE ree ed Trace. .70 1. 43 ao 
CaCQueli0 BEMIORN OIE. TOV O Denied GM Bela ||| &S) 52) VBA 3.13 
MaCO sre: ceienn ce a. peep elec. od eu. adatetice as dieey Oe Sou heake 3.00 
Pofalh 224 07, HELTOMOY JOU O56 100.45 | 100.70 | 100.68 | 100. 21 
(1) Analysis by J. P. Maider, city chemist of Spokane, probably second generation. 
(2) Average of two analyses by the writer made upon a second-generation nodule 
1 cm. in diameter from which a thin section was cut. 
(3) Analysis by the writer on second-generation nodule 2 cm. in diameter. 
(4) Analysis by the writer of several small spherules of the light-colored first-gen- 
eration siderite. 
