614 



GEOLOGY. 



crystallization while the surrounding material is yet in the fluid condi- 

 tion. In most igneous rocks nearly perfect crystals of certain minerals 



Fig. 471. — Lava flowing over a precipice near Hilo, Hawaiian Islands. 



are common, while other minerals, crystallizing later, are compelled 

 to adapt themselves to the space left. This conception is supported 

 by the fact that lavas, while still in the fluid condition, often contain 

 well-formed crystals, and these crystals sometimes make up a con- 

 siderable percent, of the flowing mass, just as water in certain condi- 

 tions may be filled with crystals of ice. So also crystals after having 

 been formed may be redissolved in part, doubtless because of changes 

 in the nature of the magma due to undetermined conditions which may 

 arise in the process of crystallization, or from the accession of gas, 

 or from new material dissolved from the walls of the passageway. 



Consanguinity and succession of lavas. — The lavas that are poured 

 forth at different stages in the succe^ion of eruptions of a given region 

 are usually not the same, as might naturally be expected, but form a 

 curious series the members of which are related to one another. Id- 

 dings has called this relation consanguinity. '^ No universal law of succes- 

 sion has yet been established, and perhaps none exists; but Richthofen^ 

 many years ago announced a definite order for the Tertiary flows of 



» Origin of Igneous Rocks. Phil. Soc. of Wash., Vol. XII, pp. 89-214. 

 * The Natural System of Volcanic Rocks. Cal. Acad, of Sci., 1868. 



