PENNSYLVANIA ANORTHOSITES 
47 
would produce protoclastic texture or granulation. This may be 
a common feature in anorthosite.^^ 
Bowen’s hypothesis meets not with entire favor by the Adiron¬ 
dack geologists. As before stated, the anorthosite of the north¬ 
ern Adirondacks is intimately associated with syenite, which Bowen 
wishes to interpret as a salic, and therefore later, differentiate of 
the same magma from which the anorthosite and underlying py- 
roxenite differentiated. Cushing admits that the Bowen hy¬ 
pothesis is the most probable that has been brought forward. He 
is also willing to concede that the syenite in the immediate vicinity 
of the anorthosite might be so considered. Miller,^® on the other 
hand, insists that the anorthosite is older than the syenite, and he 
oifers proof of their being separated by a gabbro which repre- 
" sents the chilled border of the chamber in which the anorthosite 
was formed. The anorthosite is regarded as formed by the cry¬ 
stallization of the upper, or residual, portion of an intruded gab- 
broic magma from which many of the mafic constituents have set¬ 
tled out. The anorthosite as such, is to some extent actually 
molten. Miller considers the anorthosite to be the only residual 
magma after the metasilicates sink to the bottom of the intrusive 
chamber. After approximately 90 per cent of the latter collect 
at the bottom, the magma above is a 'Tesidual” anorthosite which 
is “to a very considerable degree at least, actually molten.” 
It must, however, be borne in mind that neither artificial melts 
nor natural systems have yet proved that metasilicates crystallize 
appreciably earlier than plagioclase; as before stated, their periods 
of crystallization are very nearly contemporaneous. The former, 
when once crystallized, will assuredly outstrip the latter in rate 
of sinking because of their greater density. Yet, unless it is 
supposed that the liquid contains something more than potential 
plagioclase, could it be maintained in a sufficiently fluid condition 
to permit the mafic crystals to sink through the crystallizing feld¬ 
spars? Furthermore, without the presence of mineralizers to 
keep the liquid buoyed up and active, could the calcic plagioclase 
crystals change from Ab^ Ang to Ab^ An^ without developing 
zonal growth? In accordance with the general law of increasing 
acidity in “residual magmas,” as argued by Lane,^^ we should 
44 Journal Geology, Vol. XXV, p. 218, 1917. 
45 Journal Geology, Vol. XXV, p. 501, 1917. 
46 Bull. Geol. Soc. America, Vol. XXIX, p. 461, 1918. 
47 Journal Geology, Vol. XXX, p. 163, 1922. 
