and its Spherulitic Crystallisation. 109 



gi'eater at a given temperature in proportion to tlie amount of 

 hydration, thus permitting tlie crystalline arrangement of the 

 molecules in places of greater hydration, while tiie surround- 

 ing less hydrated portions are becoming too viscous." 



in his latest discussion of spherulitic crystallization, Iddings* 

 again refers the conditions controlling it in acid volcanic 

 glasses to varying amounts of water vapor — as " probably 

 dependent on viscosity, as affected by the gas contents of a 

 magma." In these discussions, however, the underlying idea 

 appears to be not so much an explanation of the assumption of 

 the spherulitic _/by'W, or habit of growth, as of the production 

 of local conditions which would favor er^'stallization and 

 permit the formation of one component rather than another, 

 for in spherulitic growths in the natural volcanic glasses one 

 ninst deal with quartz and feldspar. 



Spherulitic crystallization in the ultimate analysis is a ques- 

 tion of crystal form or i*ather habit. The essential thing in a 

 typical spherulite is that from a common center crystals grow 

 in all directions whose elongation is excessive as compared 

 with their breadth and thickness. They may be straight rods, 

 or branching rays, or blades, or assume arborescent shapes, all 

 of which occur in the Kane specimens, but always tending to 

 elongate forms, thickly crowded. The suggestion of Cross 

 tends to assume that the spherical shape or rather area was 

 defined before crystallization actually occurred and is thus an 

 explanation rather of the outward form than of the inward 

 structure. It seems highly probable that the degree of hydra- 

 tion in the natural acid glasses affecting the viscosity, as sug- 

 gested by the writers above, plays an important role in deter- 

 mining the conditions and places for spherulitic crystallization, 

 but in the Kane glass this agency was not present and the 

 complication of having two mineral substances to deal with is 

 also wanting. The question here is simply one of the condi- 

 tions which determined the assumption of a certain kind of 

 crystal habit, and the answer, in the writer's opinion, is to be 

 found in the degree of viscosity which had been attained at 

 the time when the saturation of the solution with the diopside 

 molecule reached the crystallizing point. Iddingsf states that 

 the habits of crystals depend in a large degree on the viscosity 

 ■oi the magma, long slender prisms and branching shapes being 

 commonly developed when it is very viscous, though he does 

 not explain why this is so. The writer offers this explanation 

 for the slender fibers in the spherulites of diopside in the Kane 

 glass. The pyroxene has a prismatic cleavage and is elongate 



* Igneous Rocks, vol. i, pp. 231, 233, 1909. 

 f Igneous Rocks, vol. i, pp. 206, 216, 1909. 



