PENNSYLVANIA ANORTHOSITES 
45 
question. Bowen maintains that for the formation of small 
dikes anorthosite should contain 15 to 20 per cent of metasilicates, 
or other constituents such as orthoclase and quartz. Miller, 
however, finds that in the Lake Placid region of the Adirondack 
Mountains, anorthosite, much poorer in metasilicates, occurs com¬ 
monly in the overlying Grenville series. Adams does not record 
the occurrence of anorthosite in the form of small dikes in any of 
the Canadian areas. It would doubtless be conceded that anor¬ 
thosite containing more than 95 per cent feldspar would not flow 
and would not appear as dikes. 
Although it is generally believed that anorthosite is a differ¬ 
entiate of gabbroic magma, few petrologists have described the 
process by which this differentiation might be effected. The 
formation of the Lake Superior anorthosite by the accumulation 
of crystals of feldspar was suggested in 1893, by Elftman.^® In 
1900, N. H. Winchell,^® in describing the same region, speaks of 
the feldspar crystals as separating from the gabbroic magma ac¬ 
cording to their specific gravity. 
Comparatively recently Bo wen,making a study of the rock by 
means of experimental work with artificial melts, especially of the 
ternary system, diopside-anorthite-albite, compares his conclusions 
with those drawn from field observation. The direct application 
of accurate experimental data to the problems of petrogenesis 
involved in mono-mineralic rocks is of great importance and in¬ 
terest. Previous to Bowen’s articles, certain peculiarities of 
anorthosite were not altogether appreciated or perfectly under¬ 
stood. 
Bowen’s chief contribution to the problem of the origin of an¬ 
orthosite is the conclusion that anorthosite as such cannot exist 
in the liquid phase.^^ Only by the separation and sorting of 
plagioclase crystals from a magma which is probably gabbroid, 
can anorthosite be formed. The process by which this sorting 
occurs may be stated briefly as follows: One must presuppose a 
magmatic chamber of sufficient size and at sufficient depth to 
allow for the retention of heat within the enclosed magma over 
36 Journal Geology, Vol. XXV, p. 242, 1917. 
37 Bull. Geol. Soc, America, Vol. XXIX, p. 426, 1918. 
38 Bull. Geol. Soc. America, Vol. XXVIII, p. 155, 19'17. 
39 Minn. Geol. and Nat. Hist. Surv., 22nd Ann. Rept., p, 178, 1894. 
40 Minn. Geol. and Hist. Surv., Final Report, Vol. V, pp. 66-67, 1900. 
41 Journal Geology, Vol. XXV, 1917. 
42/fnrf., p. 211. 
I 
