CRYSTALLIZATION OF IGNEOUS ROCKS. 
109 
which it has existed; for it has been found*, in the case of 
leucite, that simple heating obliterates the optical anomalies 
and twin lamination, and restores the mineral to what was 
undoubtedly its original crystal character. Hence the range 
of physical conditions under which igneous rocks have crys¬ 
tallized does not appear to have been great enough to have 
produced dimorphism, with the exception of the often con¬ 
temporaneous production of quartz and tridymite. 
Condition of Rock Magmas Previous to Crystallization. —Fi¬ 
nally, when it is remembered that most of the porphyrit- 
ical or older minerals in glassy rocks carry glass inclusions 
with comparatively large gas bubbles, the diameter of the 
bubble being half that of the whole inclusion in some instances, 
which is often the case with quartz ; and when it is consid¬ 
ered that these bubbles indicate either that the glass has con¬ 
tracted upon solidification and has become so much denser 
than when first inclosed in the mineral at some great depth, 
or else that gas actually existed in the magma at the time of 
the inclosure as a gas with the volume of the bubble, and 
has not condensed to a liquid state, we may reasonably con¬ 
clude that the ordinary phenomena of the crystallization of 
igneous rocks do not indicate the existence of extraordinary 
physical conditions at the commencement of the crystalli¬ 
zation of their magma, and that they throw no light on the 
possible condition of these magmas previous to the molten 
fluid state in which crystallization began. 
Conceptions of such previous conditions, if they are to be 
gotten at all, must be derived from the consideration of other 
phenomena than those exhibited by the ordinary crystalli¬ 
zation of igneous rocks. 
Resume. 
There are numerous facts concerning the crystallization of 
igneous rocks that are familiar to all students of petrography, 
*C. Klein. Neues Jahrb. Min., etc., 1884, II, p. 50; S. L. Penfield, 
ibid., 1884, II, p. 224; H. Rosenbusch, ibid., 1885, II, p. 59. 
