246 HAROLD L. ALLING 
two-phase systems coexist in equilibrium. Because of the great 
viscosity of the high alkali feldspars we can expect that solid solu- 
tions can exist as metastable systems in the regions between the 
ternary solubility lines, but with increased lime content, regions 
near An, the ability of these feldspars to undercool and remain 
homogeneous at normal temperatures decreases to a point where 
intergrowths are rare. 
The line EE, joins the eutectic points of the two eutectiferous 
binaries; and is consequently called the eutectic line. Although 
Vogt in the perspective drawing shows the eutectic line EE, hori- 
zontal, the probability is that £ is situated at a higher temperature 
than E,. We do not know the actual temperatures of the binary 
eutectics, but there is some evidence to suspect that the main 
portion of the solidus surface slopes down from £ to £,. 
Figure 11 shows the liquidus and the solidus surfaces of the 
ternary system projected to the base and represented by contours 
of equal temperatures: isotherms. These diagrams are only 
approximate, but in spite of that fact it must be remembered 
that they constitute the rather meager basis upon which our 
knowledge rests. 
Vogt pointed out that the feldspars which first crystallize from 
a magma are of different composition from those that form during 
the later stages of freezing. The early formed crystals are of the 
orthoclastic or microclinic type if the ratio of the potash component 
to the soda member plus anorthite was greater than the eutectic 
ratio. If the ratio, on the other hand, was less than that of the 
eutectic then the feldspars formed during the later stages of crys- 
tallization would be plagioclase. These phenomena have been 
indicated in Vogt’s diagram in the lower part of Figure ro. The 
little circles represent the composition of the first-formed feldspars, 
and the arrows show the direction of the change in composition 
taken by the subsequent forming minerals, the arrow heads them- 
selves indicating the composition of the last-formed minerals. 
The diagram clearly shows that during the freezing of a melt of 
eutectic composition the feldspars separate from that ratio toward 
both the potash and plagioclase areas. In other words the feld- 
spars ‘‘split along the eutectic line.” The consequences that 
