822 THE JOURNAL OF GEOLOGY. 



oxide or with granular quartz. Crossed nicols show that the 

 spherulites are oriented optically with the surrounding silica, and 

 that the preservation of the radiate structure is due to the arrange- 

 ment of impurities. The appearance of these vesicles is very 

 like those figured by Professor Cole, 1 who explains their forma- 

 tion by a dual mode of growth — a growth from the groundmass 

 outward converging toward a center, as well as from the center. 

 Whatever may be the facts with reference to the Roche Rosse 

 Obsidians, it is not necessary to call into play an abnormal 

 method of crystallization to explain the phenomena observed in 

 the South Mountain rocks. The spherulites projecting into the 

 vesicles, with their bases sunk into its wall, were recognized by 

 Professor Iddings, who kindly examined the sections, as tridy- 

 mite spherulites, such as form on the walls of vesicular cavities 

 in all modern lavas. 



Taxitic structure. — Still another structure which the South 

 Mountain rocks possess in common with rhyolites is what has 

 been called the taxitic. This consists in the intimate mingling 

 of two portions of the magma, which, from some cause (liqua- 

 tion), are slightly differentiated. The iron constituent, which 

 evidently separated out in the original glass, has been still 

 further crowded into bands and curved lines by the secondary 

 crystallization. The result is the production in some cases of 

 an irregular mottling : ataxites ; and in other cases of a more or 

 less complex network of interlacing bands following lines of flow : 

 eutaxites. This mottling and banding is rendered the more 

 striking by a marked contrast in color. The body of the rock 

 is light gray or pink, and the lines dark blue, gray or red, 

 according as the iron is more or less oxidized. When the iron 

 constituent is arranged in oval or spherical outlines, denoting the 

 former presence of spherulites, the rock may properly be termed 

 a spherotaxite . 2 



1 Grenville A. J. Cole and Gerard W. Butler : on the Lithophysae in the 

 Obsidian of the Roche Rosse, Lipari. Q. J. G. S., Vol. XLVIIL, p. 438. 



2 Note sur les Taxites et sur les Roches clastique Volcanique. Bui. de 1' Soc. Beige. 

 d'Geo. et Tome V., 1893. 



