74 B. K. EMERSON — DIABASE PITCHSTONE AND MUD ENCLOSURES. 



In another specimen of apple green glass from near the top of the 

 stratum each fragment is bordered by a band of greenish yellow glass 

 (see plate 7, figure 4), crowded full of round spherulitic spots, which are 

 red brown at center and graduate into aureoles of pale yellow, which pass 

 quickly into the greenish yellow border. They polarize brightly, show 

 black cross, but are not fibrous. They are spherulites produced by tension, 

 not by fibrous growth. The larger fibrous spherulites in the glass (see 



figure 3 and plate 8, figure 1) are 

 usually perfect circles or ovals, but 

 they are sometimes distorted by flow 

 or pressure. They are often bor- 

 dered by several concentric bands of 

 lighter and darker brownish green 

 glass, each band having a concen- 

 tric radiate structure. The central 

 part is colorless and beautifully ra- 

 diate-fibrous, showing perfect black 

 cross. The fibers are optically posi- 

 tive and polarize like a plagioclase. 

 They are not affected by boiling acid 

 or alkali. Sometimes the centers are 

 filled by a greenish granular mass, 

 which scarcely polarizes, showing 

 only scattered light points. The 

 spherulites are often broken and found in parts in the breccia, and the 

 layers separated and crushed, so that the glass seems full of fragments 

 of eggshells. Drops of red glass drawn out in threads are devitrified in 

 the same way (see plate 8, figure 2). 



A similar fibrous devitrification sometimes affects all the fragments 

 of a slide, each one being now a pale yellow devitrified glass of a finely 

 tufted or fibrous structure radiating from many centers. The fibers have 

 the same optical properties as those of the spherulites (see plate 8, figure 

 3)., The enclosing glass is more granularly devitrified, polarizing in dots. 

 Treated with acid and fuchsin, the centers of some spherulites and the 

 halo surrounding them were dissolved. The primary phenocrysts were 

 not affected. The enclosing glass was slightly affected by fuchsin and 

 bluish ; the fibrous glass was much dissolved and stained strongly. 



The feldspar rods are generally well formed crystals, and large groups 

 of sharply defined colorless pyroxenes sometimes occur together with 

 scattered olivines of equally perfect form. They often enclose blebs and 

 lobes and are themselves surrounded by a layer of a glass much deeper 

 brown than the surrounding glass. 



Figure 3. 



-Forms of Spherulites (magnified 40 

 times). 



