THE MINERALOGRAPHY OF THE FELDSPARS 228 
ores. His opinion is that the great majority of such intergrowths 
are due to secondary metasomatic replacement and not to the 
decrease in solubility of the components as they pass from the 
liquid to the solid phase. They are not eutectics in the sense in 
which the metallographer would use the term. 
C. H. Smyth, Jr.," says in regard to the microperthite con- 
tained in a ‘‘gneiss” from the Adirondack Mountains (which is, 
in all probability, the Adirondack augite-syenite of Algoman age): 
A very marked feature in a majority of the sections examined is the 
great abundance of the microperthite intergrowth of orthoclase and plagioclase. 
. .. . In most instances the microperthite has the appearance of that of a 
contemporaneous crystallization of the two feldspars; but enough sections 
contain absolute proof of its secondary nature to render it extremely probable 
that in this gneiss it is never an original intergrowth. Evidence of this second- 
ary origin is seen in the plagioclase spindles passing unbroken across cracks 
in the orthoclase and in the evident optical continuity of the material of the 
spindles and secondary feldspar filling cavities and cracks adjacent to the 
microperthite. 
Smyth gives the reader the impression that “contemporaneous 
crystallization of the two feldspars”’ and their ‘‘secondary origin” 
are incompatible. The writer would agree that the spindles could 
have formed subsequent to solidification and thus are secondary 
in this sense. Yet the material found in the two phases was 
present at the time the rock solidified. Smyth has apparently 
overlooked the question of the relative solubilities of the two 
_ phases with lowering temperature. 
The examination of a large number of perthitic feldspars in 
thin section reveals the fact that most of them did not have an 
original eutectic composition, but have assumed their present 
form through the agency of exsolution. However, on the margins 
of the grains or in the interstitial spaces, perthite feldspars are 
frequently found, and are interpreted as representing the eutectic 
mixture. An excellent example of this type of feldspar is the 
pegmatite (graphic granite) from Bedford, New York. 
Howe? makes the distinction between the eutectoid and the 
conglomerate of cementite (Fe,C) and ferrite (alpha iron) in steels. 
tC. H. Smyth, Jr., Trans. New Vork Acad. Sci., XII (1893), 204. 
2H. M. Howe, Metallography of Steel and Cast Iron (1916), pp. 71, 161. 
