ArT. 12. SIDERITE AND ASSOCIATED MINERALS—SHANNON. 15 
occurs throughout the cavities which contain the large second-gener- 
ation siderites as a thin botryoidal layer underlying the siderite. It 
also occurs interstitially in the enclosing rock, constituting a partic- 
ular and typical form of alteration or opalization. In the hand speci- 
men this opal is dark brown to black in color with greasy, pitchy, 
or velvety dull luster. Under a lens it is seen to have a minute 
botryoidal surface. It coats the earlier minerals, feldspar, magne- 
tite, cristobalite, and first-generation siderite. The dark-brown color 
‘is doubtless due to ferric hydrate, indicating that the opal was 
formed in the presence of free oxygen; but the solutions which 
deposited it in the cavities were not oxidizing, since the minute sider- 
ite globules beneath the opal layer are not oxidized, Under the 
microscope the opal is transparent, reddish brown to golden brown 
in color, and devoid of any cleavage or inclusions. It is completely 
isotropic with an index of refraction of 1.445+0.003. The material 
varies somewhat in index, however, that in the fabric of the basalt 
running down as low as 1.432 +0.003. 
‘The second variety of opal is perfectly limpid colorless hyalite which 
was deposited later than the second-generation siderite, although it 
is not confined to the cavities containing this siderite, but sometimes 
coats first-generation siderite in nonopalized light-colored basalt. Its 
perfectly limpid transparent botryoidal crusts vary from exceedingly 
thin to a thickness of 3 mm. or more (pl. 3, upper). It may occur 
coating the entire interior of a cavity, being deposited impartially on 
all of the minerals, or it may be dotted in minute limpid drops over 
the surfaces of the siderite. In one or two specimens the hyalite 
appears to be earlier than the second-generation siderite, but this is 
due apparently to its depositing over the space between the siderites 
and avoiding them. The opal is later than the aragonite in the single 
specimen in which both occur, and long acicular aragonite crystals 
bear a bead of hyalite at their termination. Under the microscope 
the hyalite is transparent and colorless. It is feebly birefringent and 
has an index of refraction of 1.455 +0.003. 
ARAGONITE. 
Aragonite occurs as a constituent of two specimens. One of these 
is a rosette of long prismatic crystals of aragonite, which shows no 
basalt attached, nor does it show any of the other characteristic 
minerals, so that its relationships are not known. ‘The crystals are 
repeatedly twinned and have obscure terminations resembling frac- 
tures. Little could be learned from a crystallographic study. The 
second specimen shows aragonite in minute acicular crystals and 
crystalline masses resting on iron opal in a cavity in black vitreous 
basalt. The aragonite is partly coated by hyalite. The crystals are 
too minute and imperfect for measurement. 
