EXPLANATION OF PLATES 129 



Figure 5. — Cavity in coarse porphyritic diabase (X 4). 



A large striated magnetite and a calcite project into the cavity, 

 which is partly bordered by black glass. The cavity itself is filled 

 by pale blue fine granular quartz (see page 102). 



Figure 6.— An arrowheaded twin of labradorite, full of grains of pyroxene and 

 glass (see page 102). 



Plate 29. — Glass- lined Cavity 



Figure 1. — Partly collapsed, glass-lined cavity (X 20). 



The cavity is in a coarse porphyritic glass-bearing diabase. It con- 

 tains tufts and a sphserocrystal of calcite and ankerite and is filled 

 with fine granular quartz. The black border is a yellow-brown 

 non- polarizing glass greatly wrinkled and broken by collapse, so 

 that isolated fragments are found in the quartz, the sphaerocrystal, 

 and the surrounding rock. It is missing in part only because the 

 whole cavity is not preserved in the slide. Delicate tufts of brown 

 ankerite crystals have formed on many projecting points, and one 

 has developed into a beautiful spruerocrystal having alternate 

 layers of white calcite. The center is filled with limpid quartz, 

 which polarizes in a multitude of black crosses and so reveajs a 

 minutely radiate texture. A part torn from the main bubble ap- 

 pears isolated below. 



Figure 2. — Central portion of figure 1 (X 32). 



The ground below is the fine grained quartz-albite halo of holyokeite 

 surrounding the glass (see page 119). 



Plate 30. — Palagonite, denitrified Glass, and Holyokeite Material 



Figure l.—Seljadalr palagonite (X 20). 



The colorless spaces filled by secondary analcite are variously col- 

 lapsed steam holes, into one of which a bent thread of glass pro- 

 jects. They are bordered by yellow fibrous devitrified glass, and 

 many similar bands appear in the fawn-colored glass, derived 

 from wholly collapsed or shattered bubbles (see page 121). 



Figure 2. — Botryoidal devitrified glass mass (X 32). 



This mass is from the coarse diabase with resorbed pyroxenes. The 

 upper third of the figure and the gray spot to the right of the 

 center is the fine grained quartz albite holyokeite. The three 

 large white spots are the finely radiate quartz-filling of the 

 irregular cavity formed by the partial collapse of the steam hole. 

 A few calcite grains mark the boundary of a perfect spherulite near 

 the center. The outer boundary of this has suffered a slight fibrous 

 devitrification. The center and the surrounding clear yellow 

 glass is mostly non-polarizing except when it grades into an agate- 

 like banded part which polarizes softly (not shown in slide). 

 Where it borders on the cavity there is a double band of lighter 

 color and even thickness, fibrous, brightly polarizing, and show- 

 ing everywhere the black cross in perfection (see page 105). 



