372 Haworth on the Archcean Geology of Missouri. 
its general appearance resembles the porhpyrics. In some places 
rocks were seen which, macroscopically, as much resembled the 
porphyries as the granites. 
If we turn to the porphyries themselves we shall find, that 
those from the base of the hills are more coarse grained than 
those near the top. Number 36S is from the base of a hill more 
than 400 feet high. It is in structure half way between the 
granites and porphyries. All of the porphyr3'-tes from number 
273 to 2S0 are from quarries which are near the bases of the 
hills. They all have a very coarse grained microgranitic struc- 
ture. Number 287, from about two miles west of the Silver 
Mines, is from the base of a hill 500 feet high. It has the 
poecilitic structure very well developed, and is a very coarse 
grained porphyry. 
In more than 220 hand specimens examined in the prepara- 
tion of this article not a single one represents a coarse grained 
porphyry from the top of a hill, while many of them show 
that the rocks at the top of high hills are much more fine grained 
than those from the bases of the same. The transition from 
the coarsest microgranite into the finest grained felsophyre is 
absolutely without a break. The series from the coarse grained 
granite to the micro-granites is almost as complete. 
No instance was seen in which this change could be traced 
from one extreme degree of crystallization into the other in the 
same rockmass. But this may be due to the insufficiency of 
the field-^vork done, or to the fact that a heavy covering of soil 
usually conceals the rocks over an area between the granites 
along the streams aud the porphyry hills farther back. 
It is thought that the evidence now at hand is suflicient to 
warrant the suggestion, as a working hypothesis, that we have 
here an example of the gradual transition from a coarse grained 
granite into a very fine grained felsophyre; such transition in 
structure being due to the difference of physical conditions — 
heat and pressure — under which the various massess solidified, 
differences largely brought about by the varying depths at 
which different portions of the original magma existed. 
The old classification of massive rocks based upon their age 
is at present almost entirely abandoned, but a short review of 
the work which has contributed to this result may not be amiss 
in this jDlace. 
