120 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



ject into the empty cavity. They are partial spheres because they rest 

 on the irregular crumpled film of glass. They seem to derive their 

 banded brown color from the slight oxidation of alternating layers of 

 ankerite, between which are transparent layers of calcite. Treated with 

 HC1 under the microscope the lighter concentric layers effervesced abun- 

 dantly as if calcite. The remaining darker layers effervesced slowly like 

 ankerite. This continued half an hour without complete solution. The 

 slide was then heated and the remnant dissolved rapidly with efferves- 

 cence. The interior of these cavities is filled by the fine> more or less 

 radiate tufts of water-deposited quartz. More interesting (see plate 28, 

 figures 4 and 5) is the case where the botryoidal cavities, often i inch across, 

 are filled with the radiating, deep cobalt blue quartz, whose radiations 

 always start from a point or points on the botryoidal surface and are 

 thus like the chondrites of meteorites. They have grown out as radiate 

 fibrous balls until they have filled the cavities. In cross-section the 

 globular projections of the glass penetrate into the quartz, and when the 

 glass is dissolved the quartz grains present an exact cast of the botryoidal 

 surface or of the spherocrystals of carbonate which have first formed on 

 the walls of the cavity. These fibrous quartz balls have formed during the 

 latter stages of consolidation while the extremely unstable glass and the 

 calcite to which they fit were perfectly fresh. If the quartz had been 

 brought in later by ordinary atmospheric waters, the calcite, ankerite, 

 and glass would sometimes show trace of corrosion. There is still an- 

 other form of quartz, found in the fresh rock, which is pale blue and fine 

 granular and forms in larger and more abundant grains (see plate 28, 

 figure 5), several quite large ones being often aggregated together. It 

 differs from the other in that it has only a little glass associated with it. 

 It often also fills veins which are of small extent, and in the solid and 

 fresh lava run out to nothing in all directions. Fissures seem to have 

 been formed in the just solidified and still greatly heated rock, and to 

 have been filled by the quartz dissolved in the superheated water in the 

 trap, but at a slightly later stadium than the blue quartz in the glass 

 cavities. This quartz is often granular under the microscope ; more often 

 the whole field is made up of the most beautiful sphasrocrystals, giving 

 the black cross everywhere. The series is complete, and the succession 

 of minerals is identical, from the glass cavities filled first with ankerite 

 sphserocrystals, followed by radiate quartz, to the great swarms of veins 

 which cement the explosion breccia down the middle of the field, and 

 are composed of bands of ankerite followed centrally by quartz often 

 quite coarsely crystalline or amethystine (see page 97). The extreme 

 purity of the calcite and quartz in the steam holes in the glass is note- 

 worthy. 



