224 HAROLD L. ALLING 
The latter is the eutectic mixture. The eutectoid marks a change 
in the solid state and is called “‘pearlite,”’ while the eutectic is 
referred to as “‘primary pearlite”’ or ‘“‘ledeburite,” a distinction 
well to make. The eutectoid pearlite is somewhat analogous to 
perthite due to exsolution. As this exsolution is a slow process 
the appearance of the two phases may not occur until many thou- 
sands of years after the freezing of the magma. ‘There would be 
a gain for the sake of clearness if the literature of petrography can 
receive the term ‘‘perthoid” to refer to intergrowths of potash- 
soda feldspars due to exsolution, while “‘ perthite”’ could be reserved 
for the true eutectic. 
Undercooling.—Hitherto it has been assumed that each new 
phase made its appearance at the temperature at which it theo- 
retically formed. In the laboratory this condition is rarely secured, 
while in nature, pressure, gases, and time are factors which pro- 
foundly affect the crystallization of a system. Inertia often causes 
a melt to remain in the liquid condition although the temperature 
may be below the freezing-point. This phenomenon is termed 
‘“‘undercooling.”’ 
The bridging of the eutectic gap by undercooling is discussed 
with the aid of the diagram shown in Figure 5. ‘Three variables 
are represented: temperature, composition, and degree of equi- 
librium; and consequently a three-dimension model is necessary 
for representation. The back plane, ABT, ET-., is: the normal 
diagram of the potash-soda feldspars, after Vogt and Warren. The 
plane nearest the reader is the metastable diagram when the two 
components A, and B, are completely soluble in the solid. The 
form assumed by the liquidus and solidus curves comes within 
Roozeboom’s classification of Type III. The analogous points of 
the two diagrams are connected. The field of the rear plane 
where two solid phases are in equilibrium, CDGOF, becomes more 
and more restricted in passing to the front plane and comes to an 
end at E,J. Stated in words, this indicates that the solubility of 
the two solid phases in each other becomes increasingly greater until 
complete solubility prevails. The converse of this is that if 
homogeneous crystals are formed upon freezing through the 
process of undercooling then because they are supersaturated in 
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