art, 12, SIDERITE AND ASSOCIATED MINERALS—SHANNON. sig 
occurred as isolated patches on the siderite spheres. Apparently the 
tendency was toward selective deposition on the feldspar in preference 
to the siderite. The pyrite forms mossy coatings which show no evi- 
dences of crystal form. It commonly shows a deep bronzy tarnish. 
SIDERITE (SPHAEROSIDERITE). 
The most conspicuous and most universally distributed mineral of 
the cavities is the siderite, which first attracted attention to the 
minerals here described. While siderite in the ordinary cleavable 
granular form is a common enough mineral, that in the basalt cavities 
is the variety which, on account of its peculiar concretionary form, 
is known as sphaerosiderite. Careful study of the specimens at hand 
“ 12 
Fias. 9-12.SIDERITE FROM SPOKANE. 9, RADIATE FORM OF FIRST GENERATION SIDERITE. 10, Com- 
MON “‘TRIGONAL HOUR-GLASS’’ FORM OF FIRST GENERATION SIDERITE. 11, CURVED CRYSTAL OF 
SECOND GENERATION SIDERITE. 12, CRYSTAL DRAWINGS IN ORTHOGRAPHIC AND CLINOGRAPHIC 
PROJECTIONS OF CRYSTAL OF SIDERITE. 
indicates that the siderite is of two distinct generations separated by 
iron opal. These may conveniently be considered separately. 
The siderite of the first generation is widely distributed on the 
perfectly fresh nonopalized basalt, where it occurs always in small 
bodies more or less thickly scattered over the plagioclase crust. In 
size these range from very minute up to perhaps a maximum diam- 
eter of 3mm. In color they are light yellowish to pistachio green, 
and they are nearly opaque with a waxy to velvety luster. Many 
of these siderite masses are almost perfect spherulites or aggregates 
of the same, but a great number also has a peculiar trigonal hour- 
glass form, in general resembling that shown in the drawing, figure 10. 
Occasionally some of these are bronze brown externally and beauti- 
fully iridescent, apparently from very slight surface alterations. 
