Haworik on the Archcean Geology of Missouri. 287 
(e). Dykes. 
In numerous places throughout this whole country dykes of 
various sizes occur, sometimes in the granite, sometimes in the 
porphyry, and, as stated by Broadhead, sometimes in the sand- 
stone. It is difficult to determine the age of these dykes. In 
nearly all cases they occur in the massive rocks in places where 
there are no overlying sedimentary ones. The presence of one 
of them in the sandstone would indicate that probably they 
were formed after it was deposited. But in the report just 
mentioned, on page 372, Norwood speaks of a dyke which is 
overlaid by the sandstone. It seems probable, therefore, that 
the great majority of them are of Archaean age, while 2:)ossibly 
a few are younger. 
These dykes are most abundant in Madison county, but are 
known in many other places. Their most common trend is 
northeast and southwest, or approximately parallel to the Ozark 
hills. The dykes vary in size from a fraction of an inch in 
diameter to more than fortv feet. It seems that they rarely if 
ever overflowed the surface; or, if so, the material has been 
carried away by erosion. 
II.- PETROGRAPHY. 
The massive rocks of Missouri mav be divided into three gen- 
eral classes or families. 1°, Granites. 2°, Porphyries and por- 
phyrytes, exclusive of the dyke-rocks. 3°, the dyke-rocks, which 
are varieties of diabase and diabase-porphyryte. They will be 
described in the order given. 
A. Granites. 
(a). Mincralogical co»if>ositio'H. 
The mincralogical composition of the granites is not at all 
complex or varied. The great mass of the rock is composed of 
quartz and orthoclase. Other minerals are always present, but 
in comparatively small quantities. They are microcline, plagio- 
cla^e, biotite, hornblende, apatite, zircon, fluorite, topaz, and 
iron-oxide — 2^^'<^t)ably magnetite. To these must be added the 
secondary minerals; chlorite, epidote, muscovite, leucoxene, and 
calcite. Sometimes the biotite is sufficiently abundant to give 
