74 
IDDINGS. 
(.phenocrysts ) cease to be prominent, and the whole rock ap¬ 
pears to be of uniform grain. 
Completely crystallized or holocrystalline rocks, that carry no 
porphyritical crystals ( phenocrysts ), always present an evenly 
granular texture, which appears differently according to the 
size of the component crystals. The finest-grained varieties 
resemble porcelain; the coarsest are like granite, diorite, etc. 
(3.) Upon studying these various structures under the mi¬ 
croscope it is evident that in the case of porphyritic rocks 
the porphyritical crystals (j phenocrysts ) in most instances were 
formed before those composing the groundmass, and belong 
to an earlier generation. This is especially true where the 
same kind of mineral occurs in porphyritical crystals and in 
microscopic ones; but it does not hold true for all large cr}^s- 
tals when compared with all microscopic ones. It is fre¬ 
quently observed that all the different minerals which occur 
as porphyritical crystals ( phenocrysts ) in a rock may also 
occur as microscopic crystals in the groundmass. There are, 
then, two generations of the same minerals in the rock. 
Where there are no porphyritical crystals ( phenocrysts ) and 
the rock has a uniformly granular texture the crystals of the 
different minerals composing the rock have a nearly uniform 
size, and there is no evidence of more than one generation of 
any one mineral. Professor Posenbusch has made this the 
distinguishing characteristic between porphyritic and even¬ 
grained rocks. 
(4.) There are other structures of widespread occurrence 
which are specially characteristic of particular kinds of rocks 
that have consolidated under particular conditions. They 
will not be included in the present discussion. Among them 
are the laminated and spherulitic structures of acid rocks. 
Component Minerals. —(1.) The crystals that make up the 
great body of igneous rocks, with all their manifold varieties, 
belong to a comparatively few mineral species. The essential 
minerals composing X W of these rocks are: quartz; the feld- 
