368 Haworth on the Archcean Geology of Missouri. 
— due to the simultaneous crystallization of the two minerals from the 
magma — all the parts of the same individual, no matter what the size or 
shape, must have exactly the same opticalorientation, and must hence 
extinguish the light between crossed nicols together. Such a structure 
is termed, according to the particular form it assumes, micropcgmatitic or 
granophyric. In the third place a single large crystal of one of the two 
constituents of the groundmass may be filled with much smaller, irregu- 
larly arranged grains or crystals of the other. This would also give the 
general effect of a finely granular structure, although it is essentially dif- 
ferent from either of the others above mentioned." 
This type of structure apparently has never received attention 
as a variety of the holocrystalline groundmass in porphyries, 
but it is admirably exhibited in the Missouri rocks. It may 
perhaps best be designated as the ■pcecilitic structtire^ being 
strictly analogous to the mottled appearance described by this 
term by Prof. Williams in certain coarse-grained peridotytes, 
where large hornblende crystals are dotted through v^ith grains 
of olivine and hypersthene.^ 
We may therefore divide the Missouri porphyries, in regard 
to the structure of their groundmass, as follows: 
I. Holocrystalline. (i) Microgranitic. 
(2) Granophyric. 
TS) Pcecilitic. 
II. Semicrystalline. (4) Felsophyric. 
III. Glassy. (5) Vitrophyric. 
T°. The microgranitic structtire: This class includes more 
than half of the different kinds of porph3a-itic rocks of the whole 
area. It is represented in the quartz-porphyries, the quartz- 
free-porphyries, and the porphyrytes. The porphyritic indi- 
viduals in it are sometimes corroded, but not so frequently nor 
so badly as in those with felsophyric and vitrophyric structures. 
The size of the individual crystals in the groundmass varies 
greatly. In the coarsest grained ones the constituents can readily 
be determined to be quartz, orthoclase, and plagioclase, the 
particular associations depending on the nature of the rock. 
From this degree of coarseness they pass into the microfelsytes 
by almost imperceptible gradations. 
2°. The granofhyric structure: This is so well known it 
1 See the Peridotites of the Cortland series. Am. Jour. Sci., (3). xxx, 
p. 30, Jan., 1886, and xxxiii, p. 139, Feb., 1887. 
