Haworth on the Archcean Geology of Missouri. 375 
of Missouri by the following scheme, the numbers used refer- 
ring to typical specimens of each class. 
iMicrogranite, 287, 347. 
Granophyre, 268, 410. 
Felsophyre, 204, 283. 
Porphyries < Quartz-free porphyry <{ Orthoclase, 213. 
„ , . ( Epidote-free, 223. 
Porphvryte \ ^^-a ^ u • ^v- 
- ^ ^ -^ I Epidote-beanng, 273. 
C. Diabase axd diabase porphyrytes. 
Under this head are included all the dyke rocks of the dis- 
trict. It is thought they are sufficiently related to be considered, 
under one general head. The dykes themselves have already 
been described, so that the petrographic discriptions may at once 
be introduced. 
(a) Mineralogical coinfosition. 
Mineralogically there is nothing new in these rocks. The 
most abundant mineral is plagioclase. Then follow augite, oliv- 
ine, ilmenite v/ith its decomposition product leucoxene, chlorite, 
iron-oxide — probably magnetite — hornblende, quartz, biotite, 
apatite, epidote, pyrite, calcite and serpentine. To this list of 
constituents glass must be added, since it forms an essential 
part of a few rocks. 
The character and mode of occurrence of the greater portion 
of these constituents do not differ essentially from those often 
described by others, and consequently need only be mentioned 
in a general way. The ilmenite, leucoxene, and iron-oxide are 
present in all the rocks, and are quite uniformly scattered through- 
out the mass. The chlorite, of course, is a decomposition pro- 
duct, and is the most abundant in the weathered specimens. 
The biotite is scarce, but in a few specimens, particularly num- 
ber 235, large individuals of it occur. Apatite, as usual, is 
scattered through all the rocks; epidote only occasionally occurs, 
and is probably always secondary. Pyrite is not often found, 
but a few of the porphyrytes have it scattered uniformly through 
the ground-mass. Calcite and serpentine are present only as 
secondary products. 
(<5) Special description of minerals. 
Plagioclase. This mineral always occurs in the form of long. 
