REVIEWS 719 
in the grain of rocks corresponding to these conditions are exceed- 
ingly important and interesting. Some of the theoretical deductions 
are as follows : (1) After the temperature at the center of an intruded 
magma has fallen about one fourth of the interval between the initial 
temperature and the marginal temperature, the rate of cooling at a 
given temperature is the same for all. parts of the sheet. (2) If the 
initial temperature (of intrusion) and conditions of cooling are such 
that a considerable time elapses before any part of the sheet reaches 
the point of solidification, the rate of cooling will be the same at all 
points, and, so far as the grain is dependent on it, there will be no 
change of grain, but the grain will be uniform from margin to center. 
(3) The hotter the dike or sheet initially, the less will be the width of 
the marginal zones of gradually finer grain; also, the hotter the 
country rock, the less pronounced will be the marginal zone of finer 
grain. (4) The time of cooling varies as the square of the thickness 
of the dike or sheet, so that a dike 200 feet thick will cool four times 
as slowly, other things being equal, as a dike 100 feet thick. (5) 
Before the center (of an intruded mass) begins to cool off, the time 
required for a given loss of temperature for any point will vary as the 
square of its distance from the margin. 
In applying the theory to the rocks examined it was found that 
some minerals, such as augite and feldspar, conformed fairly with 
theoretical requirements. In the case of sheets which have solidified 
before the center had appreciably cooled, it was found that ‘the area 
of cross sections or surface of the grains varies directly as the slowness 
Again, “ the linear dimensions of the augite patches (in 
yr} 
of cooling. 
the ophites, are directly as the distance from the margin.” 
With regard to the effect of chemical composition on grain, it is 
announced that “ other things being equal, the greater the abundance 
of its constituent molecules, the coarser the grain of any mineral.”’ 
In conclusion, Dr. Lane ventures the prophecy “that it will prove 
widely true that superficial (extrusive) basic rocks are characterized by 
an increase of grain to near the center, while deep-seated basic rocks 
have a broad central zone of nearly uniform grain.” He remarks in 
connection with the question of the effect of geological environment 
upon rocks that “it must also be remembered that the possession 
and loss of gas by diffusion follow the same laws as the possession and 
loss of the imponderable ‘caloric,’ while the possession of gas may 
greatly lower the temperature of solidification as glass.” 
