820 THE JOURNAL OF GEOLOGY. 



shape in the hand specimen and their sharply defined outline in 

 the thin section in ordinary light alone testify to their former 

 presence. With crossed nicols these boundaries become incon- 

 spicuous, and the field of the microscope shows only a uniform 

 quartz-feldspar mosaic. The crystallization within the spheru- 

 litic boundary is sometimes finer grained than that of the ground- 

 mass, or the micropoikilitic structure is present in the former 

 when absent from the latter, otherwise the spherulite is in no 

 way distinguished from the groundmass. In the case of the 

 chain spherulites the alteration is complete and universal. There 

 is, in ordinary light, an impressive similarity with the fresh chain 

 spherulites of the Yellowstone Obsidian. The same irregularly 

 scalloped outline, the same central chain of clear spherules. With 

 crossed nicols the close similarity vanishes, for in the ancient 

 rocks the radial' growth has utterly disappeared. The clear 

 spherules are composed of finely granular quartz while the sin- 

 uous border is not to be distinguished from the quartz-feldspar 

 groundmass. 



Axiolitic structure. — Closely related genetically to the chain 

 spherulites, but unlike them in being linearly radial rather than 

 centrally, is the axiolitic formation. 1 These have been described 

 in rhyolites and occur somewhat sparingly in their ancient proto- 

 types of the South Mountain. 



Rhyolitic structure. — The sections in which the axiolites were 

 observed possess a holocrystalline character, but exhibit in 

 ordinary light flow and vesicular structures, together with string- 

 ers and shreds and curved patches of a brownish red color form- 

 ing what has been called a rhyolitic structure. This latter 

 structure, which has been figured and described by Rutley, 2 

 Nordenskjod, 3 and Vallee-Poussin, 4 and on a macroscopic 



i Zirkel : opus cit. p. 167. 



2 Rutley : On the Microscopic Structure of Devitrified Rocks from Beddgelert 

 and Snowden. Q. J. G. S., Vol. XXXVII., 1881, p. 406, Fig. 1-2. 



3NORDENSKJOLD : OpUS cit, p. 5. 



4 Vallee-Poussin : Les Anciennes Rhyolites dites Eurites de Grand-Manil. 

 Bull, de L'Acad Roy. de Belgique, 3d series, Tome 10, 1885, p. 271. 



