CRYSTALLIZATION OF IGNEOUS ROCKS. 
73 
perfect glass. The glass in most cases contains crystals of 
several minerals scattered uniformly through it. The crys¬ 
tals may be microscopic or they may be large enough to be 
seen by the unaided eye. Usually both large and small 
crystals occur in the same glass. The large ones that stand 
out prominently from the mixture of glass and small crys¬ 
tals are said to be phenocrysts *; the remainder of the rock is 
called the groundmass. The glassy groundmass may be so 
filled with minute crystals as to lose the appearance of glass 
and resemble stone or porcelain, but an examination with 
the microscope will reveal the presence of a small amount 
of glass acting as a cement for the multitude of crystals. 
(2.) Lavas may also cool in such a manner that the whole 
mass becomes crystalline and no glass is left. When there 
are porphyritical minerals {phenocrysts), they are scattered 
through a groundmass wholly made up of crystals, which 
may be of microscopic dimensions or may be large enough 
to be recognized without the aid of a lens—that is, macro- 
scopically. But surface lavas seldom attain so high a degree 
of crystallization. It is oftener reached by those magmas 
which have consolidated as intrusive bodies within the crust 
of the earth. In these rocks the crystals composing the 
groundmass may be so large that the porphyritical crystals 
* These prominent crystals in porphyritic rocks are frequently termed 
Einsprenglinge by the Germans, which is a convenient collective term with¬ 
out a good English equivalent. Realizing the advantages of such a word, 
and the inconvenience of not having an equivalent in English, the writer, 
after consultation with Prof. J. D. Dana and others, suggests the term pheno- 
cryst , from <pav;to = conspicuous or eminent, and y.pvGTa\Xo§ = crystal. 
Phenocrysts would, therefore, be all those crystals in a porphyritic rock 
that stand out conspicuously from the surrounding crystals or glass com¬ 
posing the groundmass. They may be large or small; thus the term can 
be properly applied to very small crystals, like magnetite, which belong to 
the category of the large crystals, having been crystallized in the same 
period, but which are to be distinguished from the more minute crystals of 
magnetite, which are genetically related to the groundmass. The term is 
intended to be collective, like microlite, crystallite, spherulite. Its use is 
indicated throughout the present paper. 
8—Bull. Phil. Soc., Wash., Vol. 11. 
