364 Haworth on the ArchcBan Geology of Missouri. 
Pilot Knob. As we pass to the south and southeast we find 
them becoming more and more basic. 
(a) Alineralogical composition. 
The minerals which occur in sufficient quantities to form any 
considerable proportion of the rock masses are quartz, orthoclase^ 
plagioclase, and epidote. The list of accessor}' minerals is much 
greater. Some of these are original constituents, and some are 
of secondary origin. Thev are, iron-oxides of different kinds, 
chlorite, leucoxene, calcite, apatite, fluorite, pyrite, zircon, rutile?, 
muscovite, and piedmontite. It is quite probable that biotite, or 
hornblende, or augite has been present in considerable quantity 
in the more basic varieties, and that it was the source of the 
greater portion of the epidote so abundant in some of the por- 
phyrytes; but no instance was noticed in which either of these 
three was present, even in the smallest quantity. 
It is interesting to note that the general mineralogical com- 
position of the porphyries is almost identical with that of the 
granites. The absence of biotite and hornblende and augite in 
the porphyries has already been mentioned and a probable ex- 
planation given. Topaz would be expected only where fume- 
role action has existed, and therefore its presence or absence has 
but little, if any, bearing on the probable original composition 
of the rock magma. Zircon is coinparatively rare in the por- 
phyries, and iron-oxide is much more abundant than in the 
granites. Otherwise there is almost no difference in the miner- 
alogical composition of these two rock families as they occur in 
Missouri, 
(/;) Special description of julnerals. 
Iron-oxide. Little black grains of iron- oxide — probably mag- 
netite in most instances — are scattered all through ever}' thin sec- 
tion of the porphyries examined. The most common size of 
these grains is 0.05 mm, but some of them arc much larger, and 
others are very much smaller. Quite frequently the very little 
grains are arranged in lines, and when looked at with an ordi- 
nary magnifying power would be taken for threads or hair-like 
inclusions, but when examined with a high power their true 
character is revealed. 
In some localities the iron-oxide is unevenly distributed 
