[18 ECONOMIC GEOLOGY 
occurred beside the lower margin of the gabbro. Solutions 
working upward then formed the ores by partial replacement 
of the gabbro, and the deposition of veins and nodules of 
sulphides in all the rocks along the fracture planes. The 
ore was developed in the granite and fractured gabbro 
(Creighton Mine), in the greenstones (Victoria Mine), in the 
quartz-diorite (Copper Cliff), and in the quartzites (Frood 
Mine) ; in the Alexo Mine, 140 miles from Sudbury, similar 
ore was formed in serpentine. The hydrothermal formation 
of these nickel ores is shown not only by their microscopic 
structure, but by their occurrence where planes of fracture 
and shearing admitted the solutions after the intrusion of 
the quartz-diorite, the last of the four igneous rocks in the 
mining field. The ore is due to magmatic water—not to 
magmatic segregation. 
New Careponta—New Caledonia is the second nickel 
field as regards output. The nickel is in garnierite 
(Hp(NiMg)SiO,), and in the green variety has replaced the 
iron in the serpentine, and in the brown variety the mag- 
nesium. The ore occurs to the depths of 25 to 35 feet, and 
is a shallow formation ; it is partly in crusts which have to 
be broken off the serpentine masses, and sometimes as veins 
along the joints. The source of the nickel is unknown; it 
was probably not a primary constituent of the original peri- 
dotite, as so much of the serpentine is barren. 
Gap Mine—The origin of nickel ores by segregation in 
basic igneous rock has been suggested for various fields, 
of which the Gap Mine at Lancaster, Pennsylvania, is of 
special historic interest. A band of nickel ore there occurs 
along part of the edge of an intrusive amphibolite. For long 
this mine was inaccessible, but it has been re-examined by 
T. C. Phemister (Fourn. Geol., xxxii, 1924, pp. 496-510), who 
has shown that the ore was not formed by differentiation in 
the igneous rock ; for the sulphides cut across the silicates, 
and like the associated siderite, are later than the amphi- 
bolite ; the ore was formed by replacement along many small 
fractures. 
In Floyd County, S.W. Virginia, a dyke said to be norite, 
contains nickel-bearing pyrhotite and is intrusive in syenite ; 
microscopic examination (Watson, Tr. Amer. I. M.E., xxxviii, 
1908, p. 695) shows that the sulphides are in cracks in the