292 HaworUi on the Archcean Geology of Missouri. 
quartz and feldsjoar material,' the latter of which could arrange 
itself around the feldspar crystals so as to form secondary 
growths similar to those described by Van Hise.- 
A careful study of these specimens however, shows a great 
improbability of the correctness of this explanation for these 
enlargements. There is not the slightest indication of the former 
existence of the miarolitic structure, or of druses formed from 
an}' cause. We must therefore look for another explanation. 
2° We may suppose that in the process of cooling the 
feldspar crystals were formed and floated about in the mag- 
ma for some time, as the porphyritic feldspars do in the magma 
of a porphyry. As the whole mass finally solidified these 
crystals attracted around themselves portions of the remaining 
feldspar material, and thereby were subjected to a second 
growth differing from the first principally in the irregularity 
with which it progressed. 
This in reality is nothing more than a form intermediate be- 
tween two well known phenomena; viz, the ordinary zonal 
structure in idiomorphic crystals on the one hand, and the phe- 
nomena resulting from the orienting influence of porphyritic in- 
dividuals of quartz and feldspar, in the j^orphyries on the other. 
A number of different investigators have already observed 
similar phenomena. In 18S1 C. Hopfner " gave a description of 
certain zonal structures in the triclinic feldspar from Mt. Ta- 
jumbina, Peru, and according to his observations certain irreg- 
ular developments of these produce forms somewhat similar to 
the secondary enlargements here described. 
In 1882 Prof. G. H. Williams* described and figured certain 
zones around both quartz and feldspar in quartz-porphyry 
from the Black Forest, which he attributed to the continued 
growth of the ci-ystals during the effusion period. They thus 
represent the same kind of enlargements found in the Missouri 
granites. 
In 1883 Prof. Friedrich Becke,'^ in an article entitled, "Erup- 
1 See Rosenbusch's Mass. Gest., 2nd ed., p. 39. 
» U. S. Geol. Surv. Bui. S. pp. 44-47- 
• Neues Jahrb. Band 11. 1881. p. 164. 
* Neues Jahrb. Beilage-Band 2. pp, 605-607. 
^ Min. und petrog. Mitth. vol. 5. pp. 147-173- 
