ART, 12. SIDERITE AND ASSOCIATED MINERALS—SHANNON. 1a" 
LIMONITE AND GOETHITE. 
It remains only to describe the minerals formed by the alteration 
of the siderite under the influence of weathering, the ferrous carbo- 
nate being converted readily upon exposure to the hydrated ferric 
oxides limonite and goethite. These minerals form perfect pseudo- 
morphs after the original siderite nodules, the oxidation having been 
accomplished with no alteration in volume. ‘The limonite-goethite 
pseudomorphs are firm and hard, and thin sections and polished 
F1G. 13.—BARITE CRYSTAL FROM BASALT. SIMPLE Fic. 14.—BARITE CRYSTAL SHOWING 13 FORMS. 
CRYSTAL SHOWING TAPERING. FROM BASALT. 
cross sections show them to preserve the internal structure of the 
siderite, the pseudomorphs being radially fibrous. The fibers are 
composed of crystalline goethite, while amorphous limonite occurs 
interstitially between the goethite fibers. The exteriors of the 
nodules of oxidized siderite are often smooth and glossy. In no case 
was a nodule seen only partly oxidized or with a core of unaltered 
siderite, the alteration apparently proceeding very rapidly under 
oxidizing conditions. In one specimen containing several cavities, in 
which were numerous small nodules of first-generation siderite, the 
60466—23—Proc.N.M.vol.62—— 33 
