37© Haworth on the Archcean Geology of Missouri. 
poq^hyries of this area have a structure corresponding to the 
popular microf elsitic, or felsophyric. Some of them are exceed- 
ingly fine-grained, and differ from the microgranites in this re- 
spect only ; others more closely resemble those with the ptEcilitic 
structure; while still others are rocks which have many spheru- 
lites scattered through them. Many of them have the flow 
structure well developed, and it is not unusual to find associated 
in one slide the spherulites, the flow structure, and a very fine- 
grained micro-granitic structure. 
All of the rocks of this structure are very compact and have 
a flinty fracture. They are very abundant around Middlebrook,, 
Pilot Knob, and many other localities. 
5°. The vitrophyric striicttire. Rocks of this class origi- 
nally were vitrophyres, but the process of deviti-ification has 
progressed so far that at present little if any of the isotropic 
ground mass remains, yet the structure is such there can be nO' 
reasonable doubt regarding their original condition. 
About two hundred yards north of the station at Arcadia a 
large mass of porphyry is exposed by the roadside. Specimens 
302 and 203 are from this place. They have badly corroded 
quartz and orthoclase crystals, and have the flow structure well 
developed in the ground mass. Less than a fourth of a inile 
away, at the foot of the hill northeast of the Methodist church 
is another rock, number 204, which has the flow structure as. 
well developed as any modern volcanic rock. On the south 
side of Shepard mountain, about half way from the base to 
the summit, there are a number of thin, even layers less than 
two inches thick which were thought to be sedimentary rocks,, 
but which, upon examination with the microscope, proved to be 
lava flows. Number 216 is from this place. Many other lo- 
calities of similar rocks could be given, but these will serve for 
illustration. 
The rocks representing these fine tyj^es of structure have no- 
sharp lines between them, but they pass from one extreme tO' 
the other by many intermediate grades. 
A comparative study of the jDoi-phyries and granites of this 
district reveals the interesting fact that there is no sharp division 
line between the two, at least so far as can be judged from the 
hand specimens. The question naturally arises, "Do we have 
