18 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. 62. 
siderites of one cavity were entirely oxidized while all of those of 
the other cavities were perfectly fresh. 
PARAGENESIS AND DISCUSSION. 
To summarize, the basalt cavities contain several minerals, which, 
beginning with the first, may be listed in the order of their deposition 
as follows: 
1. Plagioclase, crystals and enamelliform. 
2. Magnetite. 
3. Cristobalite. 
4. Siderite (sphaerosiderite), first generation. 
5. Pyrite. 
6. Iron opal. 
7. Siderite (sphaerosiderite), second generation. 
8. Ferricalcite. 
9. Calcite. 
10. Aragonite. 
11. Hyalite. 
12. Barite. 
13. Goethite. 
14. Limonite. 
When one comes to assign a source to these several minerals, there 
develop difficulties in the shape of insufficient data, and opinions 
must be largely speculative. There can exist little doubt that the 
feldspar, magnetite, and cristobalite are high-temperature deposits 
formed in the cavities while the basalt was at an elevated tempera- 
ture and very soon after its consolidation. Careful consideration 
has led to the conviction that the small spherulites of first-gener- 
ation siderite are likewise intimately related to the cooling of the 
lava, although this mineral was probably formed at a somewhat 
lower temperature than the preceding ones. The pyrite, in view of 
its universal distribution and the fact that it does not occur in quan- 
tity, may be glossed over without discussion. With the iron opal, 
however, difficulties again become paramount. There is to be con- 
sidered not only the film of this opal in the cavities but that which 
is developed in the enclosing rock, constituting a unique type of 
alteration, opalization, of the basalt. That this opal is colored by 
ferric oxide indicates that it was formed, or at least colored, in the 
presence of available oxygen. The opalization is a local phenomenon, 
as is the presence of second-generation siderite in the cavities, the 
two being coextensive. For the present it will be sufficient to remark 
that the opal has the appearance of a percolating water deposit, and 
its source may be connected with the underlying fine-grained silt. 
The same agency which deposited the opal doubtless also deposited 
the second-generation sphaerosiderite, and it is not improbable that 
this latter represents a concentration, by solutions, into favorable 
