Origin of Pegindtite. — Crosby and Fuller. 173 
ble organic silica originally disseminated through the lime- 
stone is dissolvx^d by percolating water and subsequentl}- 
replaces the calcium carbonate of the limestones about favor- 
able centers. The silica is precipitated and accumulates in a 
highly hydrated and gelatinous form. Its volume is thus suf- 
ficient to fill the globular cavity due to the solution of the 
limestone; and, theoretically, the accumulation continues 
until the soluble silica in the tributary portion of the lime- 
stone is exhausted. The gelatinous silica then slowly dehy- 
drates and solidifies. This process begins at the surface; a 
continuous and impervious shell is thus formed; and the 
remaining silica slowly adds itself to the inner surface 
of the shell. The viscous natu;e of the hydrate does not 
allow the molecules of silica sufficient freedom of move- 
ment for the development of a visibly granular or cr3'stal- 
line structure; and the outer lAyer of the shell is, therefore, 
cryptocrystalline or chalcedonic. But a process of spf)ntane- 
ous differentiation is in progress. The separation of a portion 
of the silica renders the solution more mobile, the molecules 
move more freely, so that cr3q3tocrystallization usually gives 
way to phenocrystallization, and the inner surface of the 
geode is studded with large and perfect crystals of quartz. 
The finely crystalline normal granite forming the main part 
of a boss corresponds to the chalcedonic shell of a geode, and 
the central veins and masses of pegmatite to the crystalline 
lining. The much larger proportion of water in the ball of 
colloid silica from which a geode is evolved determines a 
greater preponderance of the coarser crystallization and a rel- 
atively larger central cavity or pocket than in the granite 
boss; while the vastly slower crystallization of the boss pre- 
vents, as a rule, even the outer part from being left in the 
semi-amorphous or felsitic condition, which would be essential 
to give a perfect correspondence with the chalcedonic outer 
shell of the geode. 
We have observed in New Hampshire, as Williams has in 
Maryland, and other writers in Europe, a perfect gradation in 
composition between veins of typical pegmatite and ordinar}' 
quartz veins. The quartz veins often intersect the pegmatite 
and are obviously newer; though it is probable that they are 
usually substantially contemporaneous with the great masses 
