26 NEW YORK STATE MUSEUM 
same origin. . . .In all cases the ores are regarded as being 
of magmatic origin — that is, the source of their material is thought 
to have been the deep-seated molten magmas, portions of which, upon 
being intruded into the overlying rocks, solidified as the various 
gneisses now constituting the principal rocks of the Highlands ridges. 
“After partial cooling of the gneisses these were in turn intruded 
by ferruginous portions of the same magma that gave them birth, 
and these intrusions were later enriched by iron-bearing solutions 
or vapors originating in the same subterranean source. In their 
transit to the surface these solutions or vapors deposited addi- 
tional magnetite in the intruded ferruginous rocks and made the 
ore lenses that now comprise the ore bodies.” 
Thus the time-honored conception of a sedimentary origin for 
both the gneisses and the magnetite of the Highlands of New Jersey 
was gradually displaced by theories involving magmatic sources for 
the ore and its associated rocks. 
About two years previous to this time Charles P. Berkey 
encountered two or three occurrences of typical granite on the north- 
east side of the basic eruptives near Peekskill, known as the 
Cortlandt series. From their consanguinity, shown by their high 
soda content, and their field relationship Berkey *’ concluded that 
this granite, which he called the Peekskill granite, represented the 
acid differentiate of the basic eruptives, which therefore exhibit 
a greater varietal range than had formerly been supposed. 
Rogers °* subsequently restudied this extremely interesting group 
of igneous rocks °° giving, in a very creditable paper, a general idea 
of the rocks as a whole and the deposits of economic value con- 
nected with them. Berkey was engaged at this time in an exhaustive 
study of the geology of the Highlands and the region both to the 
south and to the northwest in connection with the exploratory work 
being conducted by the Board of Water Supply of New York City. 
Much of this work has not been assembled for publication until 
recently, but some expressions of it appeared from time to time. He 
was one of the first geologists to recognize the existence of profound 
63 Berkey, C. P. The Acid Extreme of the Cortlandt Series near Peeks- 
kal ING VG YSewaee, ING Sy Asoe7/s, weeks, 
64 Rogers, G. S. Ceolony of the Cortlandt Series and Its Emery Deposits. 
AnnalsyN. Vo Acad) Sct. 2111 Son rom, 
65 William H. Hobbs described the northeastern extensions of this series, 
in Connecticut; see Festschrift Zum siebzigsten Geburtstage von Heinrich 
kosenbusch, p. 25, Stuttgart, 1908. 
