30 
PENNSYLVANIA ANORTHOSITES 
Compared with the great anorthositic masses of eastern Canada 
and New York State, each of which is measured in thousands of 
square miles, the occurrence at Honeybrook seemingly has small 
areal significance. This anorthosite is, however, an extremely 
pure type, with very clear-cut boundaries. Petrologic examina¬ 
tion of the rock, therefore, furnishes data bearing upon the im¬ 
portant and as yet unsolved problem of the origin of this class of 
rocks. 
Of the various contributions to the knowledge of anorthosites, 
' whether in North America or in other countries, some merely 
recognize the type or record its occurrence; others give petro¬ 
graphic description, but only in a very general way suggest an 
origin; while only few ofifer detailed theoretical discussion of the 
genesis of anorthosite, and of mono-mineralic rocks as a class. » 
The two most important occurrences of anorthosite in North 
America are those in New York State and in eastern Canada 
which, because of their especial bearing on the Pennsylvania rocks 
under consideration will be referred to in some detail later. A 
third North American occurrence of importance is that of the 
Lake Superior region. Anorthosite outcrops at intervals along 
the Minnesota shore-line of the Lake over a distance of nearly 
fifty miles. The rock, which is considered a differentiation pro¬ 
duct of the great Duluth gabbro lopolith, has been studied in turn 
by Irving, Van Hise, Leith, Lawson, Elftman, N. H., H. V., and 
A. N. Winchell, and Grout. 
In Wyoming ^ anorthosite occurs as an intrusive, closely asso¬ 
ciated with gabbro. Lindgren and Ransome ^ record the occur¬ 
rence of a dike of anorthosite in the Cripple Creek district, Colo¬ 
rado. One infers from the brief description of this highly altered 
rock that it was by no means a pure anorthosite. Bowen makes 
reference to this reported dike in a footnote in his “Problem of 
the Anorthosites,” where he states that the rock contains biotite 
and quartz, and evidently approaches a granite in composition.^ 
Lawson,^ in his paper on the Minnesota, anorthosites, mentions 
northern New Jersey among the localities where he has observed 
rocks “almost exclusively composed of an allotriomorphic granular 
1 U. S. Geol. Surv., Atlas, Laramie-Sherman Folio (No. 173), 1910. 
2 U. S. Geol. Surv., Prof. Pap, 54, p. 55, 1906. 
3 Journal Geology, Vol. XXV, p. 233, 1917. 
4 Bull. Minn. Geol. and Nat. Hist. Surv. No. 8, p. 7, 1893. 
