Haworth on the Archcean Geology of Missouri. 369 
needs no special description here. It is represented in compar- 
atively few localities in this area. Some of the coarsely crystal- 
line varieties which are as nearly granites as porphyries belong 
to this type, such as number 268, from near Hogan. It is very 
common to find a porphyritic individual of feldspar or quartz 
that has served as a nucleus around which the radiating rays of 
the granophyre are clustered. Fig. 2, pi. i, from number 410, 
the wall rocks of the big dyke at the Tin Mines, show this 
quite well. 
3**. The fcccilitic structure. In many of the fine grained 
rocks the individual grains cannot be seen vmless highly magni- 
fied, but if the section be revolved on the stage of the microscope 
while the nicols are crossed the whole field is broken up into 
different areas, each of which will change alternately from dark 
to light. A great many little specks of feldspathic material are 
scattered through the whole mass, but they seem to be entirely 
irregular in their arrangement. Fig. i, pi. i is an attemjDt to 
represent this structure as seen in number 211, taken from the 
north side of Shephard mountain, about three-fourths of the 
distance from the base to the summit. 
As the crystallization increases these different areas increase 
in definiteness, so that in the more coarse-grained varieties their 
true character can readily be determined. Fig. 3, pi. i, shows 
a coarse-grained porphyr}^, number 347, in which this structure 
is well represented. A given quartz crystal is entirely sur- 
rounded by a larger area of a mixture of quartz and feldspar in 
which the quartz is oriented with the central crystal. In reality, 
then, this is a large quartz crystal, a portion of which has a great 
deal of the feldspathic material as an impurity scattered through 
it in an irregular manner. In some cases the feldspar is well 
crystallized, and projects into the quartz crystal, as shown in the 
same figure. This, then, is the explanation for all of these areas 
into which tlie field breaks up as the stage is revolved. Each 
one represents a quartz or feldspar crystal throughout which 
the feldspar material is irregularly scattered. The degree of 
crystallization determines the definiteness of outline of each area, 
and the amount of light which passes depends largely on the 
distribution of the feldspar material. 
4". The felsophyric strticttire. A large proportion of the 
