Haivorth on the Archccan Geology of Missouri. 
291 
\>y the original crystal, and therefore would all extinguish alike, 
and when in the process of growth they finally coalesced they 
would form but one individual. It is perhaps not improper to 
look upon the rays of the microjiegmatite border as being crys- 
tal skeletons produced under conditions unfavorable for the 
completion of the entire individual crystal. 
These enlargements have an important bearing on the question 
of the structure of the granites in which they occur. The rocks 
from which the figures already refer- 
red to were taken have every appear- 
ance of being true granites. Num- 
ber 302 is from a specimen picked 
up at Knob Luck, the shipping point 
for the Syenite quarries, from which 
place it is reported to have come. 
The specimen has a uniform text- 
ure, is of medium coaiseness, and is 
composed of a large amount of a 
light -colored feldsj^ar, but little 
quartz, and an unusually large pro- 
portion of biotite for the Missouri 
granites. Number 364 was sent 
me by Mr. Payne, and was reported 
to be from section 3, T. 33, N. R. 5 
E. It is reddish granite, of uniform texture, and is tolerably fine 
grained. The most noticeable microscopic feature of it is the 
strong resemblance in shape of the original orthoclase crystals 
to the lath-shaped feldspars in diabase. 
The original crystals are usually much more decomposed 
than the secondary zones. Other slides show the same kind of 
enlargements, but these two are the best examples. 
Two different explanations for these phenomena might pos- 
sibly suggest themselves. 
1° We may suppose the granites possess the "miarolitic" 
structure; that is, they so contracted in cooling that numerous 
cavities were formed throughout the mass. It would then be 
possible for certain crystals to project into such cavities form- 
ing a druse. Subsequently the cavities might be filled with 
Fig. s. 
X 40 diameters. 
From a granite No. 302. This 
shows an idiomorphic ortiioclase 
partly surrounded by a second- 
ary growth, and partly by a mi- 
CTopegmatite in which the feld- 
spar is attached to the original 
crystal and oriented with it. 
