SPHERULITIC CRYSTALLIZATION. 447 
and the point of solidification of the magma. In the portion 
of the magma which solidified as glass the requisite mobility 
was wanting. In that which formed crystalline spherulites 
it must have been present. It is easily demonstrated that 
absorbed water-vapor makes molten glasses more fluid, and 
also lowers their melting point or point of solidification. 
Hence we may conclude that the influence of the absorbed 
water-vapor is to render the molecular mobility of the molten 
magma greater at a given temperature in proportion to the 
amount of hydration, thus permitting the crystalline arrange¬ 
ment of the molecules in places of greater hydration while 
the surrounding less hydrated portions are becoming too 
viscous. 
In this connection the writer wishes to call attention to 
the speculations of Charles Darwin * on the lamination of 
volcanic rocks of the “ trachytic series.” After considering 
the different degrees of crystallization in the various layers of 
laminated obsidian and of what was then termed “ trachyte,” 
but which is mostly lithoidal rhyolite, and correlating them 
with the parallel layers of gas pores in pumice and obsidian, 
with a remark that “ numerous facts, as in the case of geodes, 
and of cavities in silicified wood, in primary rocks, and in 
veins, show that crystallization is much favored by space,” 
he concludes : “ That, if in a mass of cooling volcanic rock, 
any cause produced in parallel planes a number of minute 
fissures or zones of less tension (which from the pent-up 
vapors would often be expanded into crenulated air-cavities), 
the crystallization of the constituent parts, and probably the 
formation of concretions, would be superinduced or much 
favored in such places, and thus a laminated structure of the 
kind we are here considering would be generated.” As a 
possible cause for the production of parallel zones of less 
tension in volcanic rocks he cites Forbes’ theory as to the 
lamination of glacier ice and suggests a possible parallelism 
between the two groups of phenomena. 
* Darwin (Charles). Geological observations, etc. 8°. London, 1851, 
Part II (on volcanic islands), pp. 65-72. 
