The Pre-Cambrian Structure of Missouri 121 
augite (11). The diabase at Tin Mountain contains much glassy 
material(28). Most of the dikes in the Mine La Motte quad- 
rangle as described by Keyes, are olivine diabase, but one west 
of Silver Mine contains phenocrysts of quartz and orthoclase(28) 
and is therefore not a diabase. According to Haworth (53), the 
dikes of the Iron Mountain quadrangle are mostly olivine dia- 
base which varies in texture from glassy to as coarse-grained 
as the granite. Most of the dikes are pophyritic, with pheno- 
crysts as large as two inches in diameter in the case of the dike 
on the north side of Mount Devon. 
Extrusive rocks. The extrusive felsites are mostly rhyolite 
with some andesite, dacite and trachyte. Most of the rocks are 
porphyritic and many show flow structures. Flow breccias are 
found at a number of localities. The extrusives vary in color, 
occurring mostly in shades of red although there are also black, 
gtay, green, brown and purple tones. 
Evidence that these rocks are surface flows is seen in the 
following facts: pyroclastics are associated with them; columnar 
jointing is seen at Hughes Mountain; there are large amounts 
of flow breccias; a few examples of amygdules occur ; and much 
of the felsite is apparently devitrified volcanic glass. 
Pyroclastics. Pyroclastics, consisting of tuffs and agglomer- 
ates, occur in small bodies interbedded with the felsites. ‘The 
largest outcrop is near the top of Pilot Knob where the tuffs 
contain ripple marks and have been highly altered by pre-Cam- 
brian iron-bearing solutions. Above the tuff there is a thick 
bed of agglomerate. A good exposure of tuff is seen along 
Highway 21 on the south slope of Tip Top and both tuff and 
agglomerate occur on the East fork of the Black River at John- 
son Shut-in (11), Agglomerate is found in large quantity at the 
intersection of Highway 61 and the Saco road. There are also 
a large number of outcrops of material, which may be an agglom- 
frate, in the northwestern quarter of the Coldwater quadrangle. 
Tuff is found as float throughout the St. Francois Mountains 
and has been reported by Bridge (3) from Shannon County, 
