LAMINATION OF EHYOLITES. 425 



condition of the rock shows itself in layers of compact glass full of micro- 

 lites which alternate with layers full of gas cavities and with little or no 

 microlites. And in the highly pumiceous part of the flow it is seen that 

 the inflation of the glass is more marked in some layers, while there are 

 spots in which the inflation is specially pronounced. These differences 

 in the pumiceous parts of the rock are due to variable amounts of water 

 vapor in the layers of the lava; and similar differences must have existed 

 in lower parts of the same lava sheet. Moreover, the kinds of minerals 

 crystallized are those whose formation is known to be aided by the pres- 

 ence of water vapor and other vapors. Further, the localization of more 

 abundant water vapor, which gave rise to specially inflated spots in the 

 pumice, is undoubtedly the cause of the crystallization of isolated spheru- 

 lites in the compact glass. 



The greater frequency of lamination and localized crystallization in 

 acid lavas as compared with basic ones is a consequence of the generally 

 greater viscosity of acid lavas at the time of then- eruption. The basic 

 rocks have a considerably lower melting point and are much more liquid 

 up to very near the temperature of solidification. Hence diffusion would 

 take place more rapidly and the magma would be more homogeneous, 

 other things being equal. The heterogeneity of the acid lavas, so far as 

 known, is confined to the distribution of vapors, presumably of water, 

 and suggests that the water thus irregularly disseminated has not existed 

 within the magma long enough to become uniformly diffused. It must 

 therefore be looked upon as water absorbed near the earth's surface. 

 Whether there may also have existed water vapor in the magma having 

 a much longer connection with it, is more difficult to demonstrate, though 

 it is highly probable. 



