236 HAROLD L. ALLING 
The reason why it is rare in nature is not far to seek; a rock 
high in potash and lime but low in soda is rare, and when found 
the two feldspathic phases occur as separate and distinct grains. 
This can be explained on the basis that melts of the potash-lime 
feldspars, especially when rich in the latter component, are much 
less viscous than the potash-soda ranges. Consequently their 
separation into distinct identities is much more common, and 
intergrowths are rare. That the latter do occur, however, the 
writer feels convinced. 
In the Adirondack Mountains the anorthosite and the augite 
syenite, both of Algoman age,’ occur in igneous contact with each 
other with such field relations as to indicate that certain batho- 
lithic masses of these two rocks invaded the country rocks (chiefly 
Grenville sediments) at about the same time, although the anor- 
thosite is probably the older of the two in all cases. Magmatic 
assimilation of the syenite by the anorthosite has taken place to 
some extent in zones where they adjoin one another. The feldspar 
of the syenite is microperthite (antiperthite) (hyperperthite) while 
that of the anorthosite is acid labradorite (Ab,;An,,Or;). In the 
syntectic rock, marking the zone of contact, microscopic examina- 
tion reveals intergrowths of potassic feldspar and labradorite or 
orthoclase with an appreciable amount of lime (‘‘lime-orthoclase”’) 
holding blebs of either labradorite or bytownite. It is obvious 
that such rocks are not “typical” and that the process of the 
development of these potash-lime feldspars is more involved than 
the simple freezing of a magma and consequently these cannot be 
pointed to as good examples of this rather neglected family. 
But to return to more normal rocks, we find that the feldspathic 
content of most granites, syenites, monzonites, etc., is not limited 
to one species of feldspar. Even a hasty petrographic study of 
slides of these rocks shows “‘orthoclase and plagioclase.” In the 
monzonites and granodiorites both alkali and plagioclase feldspars 
are present. Occasionally a basic representative of these carries 
in addition to the potassic or alkali feldspars, basic plagioclase, 
™H. L. Alling, ‘‘Some Problems of the Adirondack Pre-Cambrian,” Amer. 
Jour. Sci. (4), XLVIII (July, tog), 62; ‘‘Geology of the Lake Clear Region,” N.Y. 
State Mus. Bull. 207-8 (1919), pp. 119-20. 
