Origin of Phaiocrysts. — Crosby. 305 
velopment of the porphyritic structure in the contact zone 
of a bathoHth it would be a natural rejoinder that a stagnant 
magma at a great depth in the earth would possess just this 
extremely slow rate of cooling. This criticism, pointed 
though it be, loses sight of the main factor — the high viscosity 
of the congealing magma, as conditioning and limiting molec- 
ular flow. Whether the rate of cooling is to be regarded as 
rapid or slow depends upon the viscosity of the magma. 
Becker* has done geology an important service by calling at- 
tention to the extreme slowness of molecular dififusion in 
highly mobile and relatively non-viscous aqueous solutions. 
The viscosity of a magma, under any conditions, is vastly 
greater than that of an aqueous solution; but at the temper- 
ature of solidification it is almost incomparably greater. There- 
fore a rate of cooling which would be extremely slow for an 
aqueous solution might be extremely rapid for a crystallizing 
magma. Fineness of grain, the rate of cooling being equal, 
is undoubtedly a fair index of relative viscosity; and the same 
conditions which would yield a merely cryptocrystalline or 
microcrystalline structure in a magma might be expected to 
afford crystals several inches or feet in diameter in an ordin- 
ary solution, as is so fully demonstrated by everyday compari- 
sons of dikes and mineral veins. Plutonic rocks exhibit a 
wide range in texture; and when the rock is finely crystalline 
or microcrystalline we may fairly say that the rate of cooling 
was rapid for a magma, although it may have been centuries 
in crystallizing. On the other hand, a coarsely macrocrystal- 
line granular texture, like that of the normal granite of the 
Blue hills, indicates extremely slow cooling even for a magma. 
The porphyritic texture, however, corresponds to an interme- 
diate rate of cooling. 
In the first case, the rate of cooling having determined a 
relatively narrow spacing of the primary centers of crystalli- 
zation, may be said to have overtaxed molecular fiow from the 
beginning of the crystallizing process and the rock consists 
wholly of the groundmass. In the second case, with a wider 
spacing of the primary centers, there was, relatively speaking, 
no overtaxing; and the rock may be said to consist wholly of 
overgrown phenocrysts, mutual interference having made 
*Amer. Jour. Sci.,vol. 153, p. 21-40. 
