The Pre-Cambrian Structure of Missouri 127 
line indicating that the contacts are not vertical but dip steeply 
enough to be partly independent of the contour lines. This 
suggests that the outcrop of granite porphyry represents the top 
of a stock. This is also indicated by the area of felsite sur- 
rounded by granite porphyry on High Top Mountain which may 
be a roof pendant, since the strike of its contacts is independent 
of topography, thus showing that the body of felsite has ver- 
cal sides. 
None of the granites have any visible internal structure 
such as parallelism of inequidimensional grains or oriented in- 
clusions. Quarrymen claim that the granite splits easiest in one 
direction and hardest in another but, in every case, the easiest 
Way is parallel to a set of joints and the ease of splitting may 
be due to incipient jointing. The lack of strongly developed, 
internal structure indicates intrusion by quiet stoping as any 
pressure should be reflected by the presence of internal structure 
in masses as small as those at Graniteville and east of Ironton. 
The basic rocks are mostly in the form of dikes which cut 
the felsites and granites. At Tin Mountain and Skrainka Hill 
there are bosses of diabase. In Little Tom Sauk Mountain in 
the Edgehill quadrangle, the diabase is apparently in the form 
of a sill, for it outcrops on the slope of a hill roughly parallel 
to the contour lines (11). 
Many of the extrusive rocks show flow structures which 
usually take the form of nearly horizontal sheets. In some 
cases, the sheeting is vertical but this is due to tilting rather 
than to initial position, Occasionally, the flow structures are 
Schlieren, which are lenticular or spindle-shaped bodies. These 
are elongated in the direction of flow, indicating the contour of 
the surface over which the lava flowed. 
As the tuffs are bedded, they show dip which may be in- 
itial or may be due to folding. An unconformity occurs in the 
tuff at Tip Top where a four inch bed of green tuff with a 
high angle of dip intersects the more gently dipping beds of 
_ brown tuff. This is probably a very local feature. 
