BOTRYOIDAJ/ GLASS l05 



filling of calcite or blue quartz. Figure 2 on plate 30 shows such a botry- 

 oidal cavity, where, within the thickness of the glass itself, is a distinct 

 spherulite in which the few concentric bands are marked by a darker 

 parting and by a few original calcite crystals. The spherulite and adja- 

 cent glass are wholly non-polarizing, but grade into a portion not visible 

 in the slide which is beautifully banded like an agate in a few broad 

 layers, concentric with the botryoidal interior and faintly fibrous and in 

 this state polarizing distinctly but weakly. It is lined by a very regular 

 double band of a lighter and completely fibrous substance in slightly 

 radiated tufts, often raveled out inwardly and polarizing more brilliantly 

 than the banded glass. The fibers in both varieties of glass are positive 

 and give a perfect coarse black cross. This brightly polarizing inner band 

 may be thought to be an immediate alteration of the glass for a certain 

 distance inward, produced by the absorption, before the deposition of 

 the quartz, of the H a O whose expansion caused the cavity. This is pos- 

 sible because both in this rock and the long plumose variety the magne- 

 tite, which was the earliest to crystallize and which in fresh pieces incloses 

 clots of glass, here incloses particles of the same size and shape of the green 

 fibrous, brightly polarizing substance.* This lighter double band lines the 

 cavity perfectly in all its windings to near one end, where it stops suddenly 

 as if ruptured, and the remaining wall of the cavity is of the darker glass, 

 but fractured and irregular. Where the section cuts the globular pro- 

 jections it shows the black cross beautifully. Outside is a broad layer 

 of the microlitic quartz-albite ground which I have below described as 

 holyokeite. The whole is inclosed in the angular space between several 

 of the large feldspar and pyroxene crystals, and is an isolated portion of 

 that which makes up the whole ground of the rock in the porphyritic 

 variety above described (see page 101) and in the holyokeite dikes. 



SPHERULITES, SPHJEROCRYSTALS, AND L1THOPHYSJE IN THE GLASS 



The glass may inclose (1) small perfect crystals or (2) crystal groups 

 and sphserocrystals of calcite, or (3) of calcite and ankerite, or (4) beauti- 

 ful spherulites with alternating layers of calcite and the black glass, or 

 (5) small masses of fine granular pale blue quartz, or (6) sphserocrystals 

 of richest cobalt blue quartz, fibrous and eccentrically radiate, or (7) 

 any combination of the above forms (see plates 28 and 29, with descrip- 



* Several delessites and chloropals occurring in basalts have a composition very close to that ot 

 the glass. Much may be said in favor of the opinion that the so-called diabantite or delessite in 

 the Triassic traps hereabout may be in large part altered palagonite. The diabantite always 

 appears in the weathered rock just where the palagonite would be devitrified. On the other hand, 

 the trap when as much or even more altered often fails to show a trace of the diabantite, which 

 may be thought to be because the trap when fresh did not contain palagonite. The cavities in 

 the Icelandic palagonites are also always lined by a fibrous layer which polarizes brightly (see 

 Page 122). 



