66 The American Geologist. February, 1905 
poured forth, or squeezed in, was under uniform conditions 
of liquefaction, temperature, pressure and absorbed mineral- 
izing gases. We shall, at the close, consider some modifica- 
tions of the phenomena which may be assigned to non-uni- 
form conditions. 
One important condition of the coarseness of a given 
mineral is the rapidity with which it forms. The slower the 
formation the coarser the grain. This in turn will obviously 
depend in some measure upon the ease with which its en- 
vironment (country rock, air or water) can absorb the 
liquefying factors (heat, or mineralizing gases, etc.) Thus 
a lava poured forth beneath the ocean may be expected to 
lose heat faster than one intruded into dry sand. We try 
to allow for these factors by assuming various widths of 
contact zones. 
The coarseness of grain of a rock is not a mathematical 
conception. We may better speak of the coarseness of 
grain of an individual constituent, the feldspar or augite or 
quartz of a rock. The general impression of the coarseness 
of grain of the rock as a whole may be due to the composite 
efifect of one or two or three of the constituent minerals. 
Usually the grain of the feldspar is an important factor. In 
the plate of drill cores (iv.) the mottling is, however, due 
to the ophitic augites, which on fresh fracture give a luster- 
mottled effect. At limes also two or more minerals, may 
vary in the same way, and we may speak of the grain be- 
coming coarser for all of them and so for the rock which 
they make up. 
Now in our mathematical discussion w^e call the condi- 
tions of consolidation (u), those of the igneous rock as it is 
poured forth or thrust in (k/^^) and those of the country 
rock zero (o). 
(A.) If the conditions of consolidation are nearer the 
former (h./^v) than the latter, the grain at the margin is finer, 
theoretically indefinitely fine. The igneous rock will have 
a glassy or aphanitic "saalband." selvage or wall. This we 
know is the customary thing for common trap dikes. The 
grain will increase at first from the margin at a uniform rate, 
not dependent on the thickness of the country rock nor of 
the contact zone. 
