Pirsson — Phenocrysts of Intrusive Igneous Pocks. 279 



point where the insolubility and crystallizing moment of a com- 

 pound begins to the increasing viscosity. Suppose a fluid rock 

 magma, without crystallizations in it, comes to rest and the 

 temperature begins to fall. At a certain point compounds, the 

 ferro-magnesian ones let us say, commence to separate. The 

 fall has been so gradual that only a few centers of crystalliza- 

 tion have been set up. The magma retains its fluidity for each 

 period and each mineral has a considerable time in which to 

 grow, orienting and assimilating its substance around it or, to 

 coin a phrase, it has a long crystallization interval. It is theo- 

 retically possible that if the conditions are stable for a long 

 enough period each mineral in turn may exhaust itself in the 

 magma before the next begins. Usually there are overlaps. 

 Thus the process goes on until each has had its turn, the rock 

 has crystallized and the granular structure is formed. 



Other things being equal, the granularity depends on the 

 length of the crystallization interval. In the case of the very 

 siliceous magmas we must take into account another factor, the 

 viscosity, which depends greatly on the included water vapor. 

 If this is present in large amount the fluidity of the magma is 

 enormously increased, and with it the radius of action of each 

 crystallizing center, that is, the crystal has a free chance to 

 grow, orient material and expand. On the other hand, if there 

 is little or no water vapor present or it escapes before crystalli- 

 zation begins, the viscosity is greatly increased, the radius of 

 action of each center is small, and, although the crystallization 

 interval may be sufficiently long, the crystals are restrained in 

 their growth, new centers are set up and the rock is fine- 

 grained. Hence we may have coarse granular and fine granu- 

 lar rocks without porphyritic structure."* 



In the case of the basic rocks, the included water vapor has 

 no such function, their viscosity increases at a vastly slower 

 rate for each degree in fall of temperature, hence the radius of 

 action for equal crystallization intervals is much larger than in 

 the acid rocks and we therefore find them quite coarse granular 

 under the same conditions which have produced fine-grained 

 structures in the acid ones. Other things being equal, the 

 granularity depends on chemical composition, but there is also 

 a limit to this, for it also depends on the number of crystalliza- 

 tion centers which have been set up. 



Porphyritic structure. — We can now consider the por- 

 phyritic structure. Suppose that an acid magma is injected 

 with included water vapor and that the fall of temperature is 

 comparatively rapid. The vapors escape rapidly and with the 

 cooling and their escape the viscosity augments in a greatly 



*See also Iddmgs' Crystallization of Igneous Rocks, Bulletin Phil. Soc, 

 "Washington, vol. xi, p. 105, 1889. 



