108 
IDDINGS. 
distributed through a magma, however, its influence may be 
overlooked. 
(4.) The influence of pressure upon the crystallization of 
magmas, judged by the evidence presented by the phenomena 
of crystallization, appears to be slight; for it has been ob¬ 
served that all of the rock-making minerals, including the 
earliest to crystallize, may form under very great pressure, 
and also under comparatively slight pressure, being devel¬ 
oped in some instances on or near the surface of the earth, 
and it is not possible to determine from the character of the 
crystals themselves whether in a particular case they were 
formed near the surface or thousands of feet below it. 
Portions of the same magma that have crystallized a thou¬ 
sand or more feet from one another vertically exhibit the 
same minerals and structure where the rate of cooling was 
about the same. 
But it has already been remarked that the influence of 
greater pressure on deeper portions of a magma may offset 
the influence of slower cooling and equalize the crystalliza¬ 
tion at different depths in a column of magma. In this case 
the influence of pressure on the crystallization of rocks at 
different depths may easily be miscalculated, as the rate of 
cooling in any case is entirely conjectural. 
With the exception of tridymite, which is almost exclu¬ 
sively confined to surface lavas, and whose development is 
probably due to mineralizing agencies and is often accom¬ 
panied by the crystallization of quartz, there is no dimorph¬ 
ism among the rock-making minerals. Their essential 
crystallographic characters are the same, whether they were 
formed early in the history of the magma or late. 
There are optical anomalies in certain minerals, such as 
leucite, which indicate that the crystal form or character 
assumed at the time of crystallization is not that which is 
stable under present conditions; but this is equally true of 
those crystals that were formed at great depths as of those 
that were formed much nearer the surface of the earth, and 
appears to arise from the change in the temperature under 
