MAGMATIC ALTERATION OF CERTAIN MINERALS 265 
pressure and slow cooling, while conditions of slight pressure 
and slow cooling are extremely favorable, and again very rapid 
cooling and little pressure are unfavorable. The last set of con- 
ditions shows that the operation requires time, the crystals 
remaining unchanged because solidification, or else cooling below 
a temperature necessary for a molecular change, took place 
before alteration had time to set in. Our final conclusion, then, 
is that a diminution of pressure, together with a high temperature 
continued for some time, are the conditions necessary for the 
alteration. 
Theory proposed.—This statement of conditions is based on 
such a large body of observations by the best petrographers on 
all classes of igneous rocks and from such a great variety of 
localities, and the exceptions to it are so few, that it seems to 
me we can safely accept it in trying to frame a theory to account 
for the alterations. Such a statement, by showing us the con- 
ditions under which the phenomena in question occur, points the 
way toward their explanation. The explanation of the alteration 
now proposed rests at bottom on the chemical nature of horn- 
blende and biotite. 
As has been briefly noted on page 257, and as may be seen 
on reference to any mineralogical handbook, these two minerals 
are much more complex in their molecular structure than pyrox- 
ene, one of the consequences of this complexity being, as is also 
indicated by experiment, that great pressure is necessary for 
their formation in an igneous magma," with probably the pres- 
ence of certain mineralizing agents.? That the latter are present 
in the magma is indicated by the content of hydrogen and flu- 
orine in them in greater or less amounts. 
This idea as to the conditions of formation of the two min- 
erals, which Siemiradzki3 was apparently the first to propose, 
* The cases of uralitic hornblende and secondary biotite due to meteorological 
or dynamical agencies are not included in this statement. Only their formation in 
igneous magmas is referred to, and it is well known that a mineral may be formed in 
several quite different ways. 
2 Cf. LEvy, Structures des Roches Eruptives. Paris, 1889, 90. 
3 SIEMIRADZKI, Neu. Jahrb. Bd. IV. 307, 1886. 
