Coarseness of Igneous Rocks — Lane. 69 
coarseness of the margin being pronounced only as the con- 
ditions of its consoHdation approach being balanced just 
half way between those of the country rock and initially in- 
jected magma. 
(C) If the temperature and conditions at which the 
molten rock comes to rest are not very near or within the 
range of conditions of consolidation there will be back from 
the margin and near .to the center a zone of uniform grain. 
This will not be all formed at the same time, but the molten 
rock will be cooling down at the same rate when it gets 
down to the same temperature. Tbis is a result of very wide 
r.pplication. The proportion of this zone of uniform width 
depends on the relations of the three temperatures, those of 
bet^inning of consolidation and of the country rock. The 
nearer the latter is to that of consolidation the broader the 
belt of uniform grain. The broader the contact zone the 
more extensive the belt of uniform grain, as might be ex- 
pected, for the broad contact zone means one that absorbs 
relatively little heat and will make the difference between 
the margin and center in conditions less. 
The formula for the grain through this center belt is so 
simple that we shall venture to give it as : 
Which being interpreted is as follows : 
The coarseness of grain (E) in the belt of uniform grain 
is proportional to the ease of crystallization and the mole- 
cular abundance of the mineral concerned (k), -and to the 
size of the mass of rock whose temperature varies consid- 
ered as a sheet of indefinite extent and thickness (c), and 
inversely proportional to the square root of the difTusivity 
(a^), and the difference between the temperature (and other 
conditions) of consolidation and those prevailing in the 
country rock (u). 
It will be noticed that in this formula the initial tem- 
perature and conditions of the molten rock do not enter, 
and play no role. These must be studied at or near the 
contact. 
We have said that this central zone of uniform grain is 
not all formed at the same time. \ow if in what would 
otherwise be such a contact zone there is a concentration 
