108 L. V. Pirsson — Artificial Lava-Flow 



These plumose fdnvis, M-liicli arc of remarkable delicacy and 

 perfection, resemble the growths of an<>;ite in basaltic glass 

 from Hawaii, described by E. S. Dana*; they differ from the 

 growths in the pitchstone of Arran in being much larger, more 

 perfect, and in tbe curved or curled character of the stems, 

 thus reseml)ling ])lumes rather than ferns. They never form 

 complete spherulites, though in places the thickly-grouped, 

 feather-like bunches partly resemble them. 



Spherulitic Crystallization. 



The vast majority of the natural spherulites occur in acid, 

 that is to say, siliceous volcanic glasses, and are composed of 

 quartz, or feldspars, or of these two minerals in vai-ious pro- 

 portions. The reason for this is because it is especially 'in 

 magmas of this nature that the relation between viscosity of 

 magma and ci-ystal growth, which is necessary for spherulitic 

 crystallization and which is discussed later, is apt to occur. 

 Spheriilite crystallizations may occur in basic magmas and are 

 known in the rocks called variolites,f but so far as the writer 

 has been able to discover, no discussion of spherulites, as such, 

 has been made which was not primarily based on material of 

 the kind referred to. It is natural, therefore, that in these 

 discussions the chemical character of the magma involved and 

 the nature of the component minerals are largely held respon- 

 sible as determinant factors for spherulitic crystallization. 

 Since such natural magmas contain, as is well known, volatile 

 constituents, especially water vapor, a considerable role has 

 been ascribed to its agency in this connection. Thus Cross^ 

 ascribed the origin of the spherulites in a rhyolite studied by 

 him to the presence of a colloidal condition in the magma, due 

 to the antecedent formation of masses of opaline silica contain- 

 ing the elements of feldspar, which caused their formation 

 and globular form. Iddings§ also, in his earlier discussions of 

 spherulites, suggests that water vapor plays an important role 

 in rendering certain places in the magma less viscous and 

 therefore more susceptible to molecular movement and crystalli- 

 zation. This would explain the formation of crystalline spher- 

 ulites in some places, while the surrounding magma solidified 

 as glass although at the same temperature. He says : " Hence 

 we may conclude that the influence of the absorbed water- 

 vapor is to render the molecular mobility of the molten magma 



* This Journal, xxxvii, 441, 1889. 



f Pirsson, Petrog. of Igneous Rocks of Little Belt Mts., 20tli Ann. Eep. 

 U. S. Geol. Surv., Pt. Ill, p. 533, 1900. 



X Constitution and Origin of Spherulites in Acid Eruptive Rocks, Bull. 

 Phil. Soc. Wash., xi, 411, 1896. 



