ORE DEPOSIT THEORIES. - a9 
eruptive rocks. It occurs in perfectly fresh undecomposed 
rocks under such microscopical relations with regard to the 
other component minerals that there can be no doubt as to 
its primary nature. Occasionally iron pyrites is replaced 
by copper pyrites. The minute particles of copper pyrites 
in the younger lavas of the Albano Mountains in Italy is a 
case in point. Pyrrhotite often occurs as an accessory 
mineral of many gabbros, and it has also been noted in 
basalts, for example, on the west coast of Greenland. 
In metallurgical processes, also, the formation of crystals 
of copper pyrites has been observed in silicious slags, which 
are very slowly cooled. Thus, the production of deposits 
of sulphide ores by the process of magmatic differentiation 
appears to be not only possible, but highly probable. 
J. H. L. Vogt has given a very exhaustive demonstration 
of the eruptive origin of a very important class of nickel ore 
deposit, the nickeliferous pyrrhotite of Sudbury (Canada), 
of Ringericke, Bamble, &c., in Norway, of Klefua, in 
Sweden, the Gap Mine (Pennsylvania), &c. These well- 
known deposits, which have a world-wide distribution, 
contain the following characteristic minerals :—Nickel- 
iferous pyrrhotite, with from 2 per cent. to 12 per 
cent. of nickel and cobalt; iron pyrites sometimes rich 
in cobalt; pentlandite (Fe,Ni)S., millerite, &ec Copper 
pyrites and titanic ore are always present, while other 
ores, such as galena, zinc-blende, compounds of arsenic 
and antimony, are either entirely absent, or are present 
only in minute traces. Both in composition and in 
the general nature of the occurrences these deposits 
exhibit essentially the same characters wherever they have 
been met with. The nickel content varies from 2 per cent. 
or 3 per cent. up to 12 per cent. The proportion of nickel 
to cobalt varies from 4:1 to as much as 15-1, and the pro 
portion of nickel to copper from 2:1 to about 4-1. They 
are always intimately connected with a gabbro rock, and in 
the latter the sulphide ores are alwaysto be found in greater 
or lesser quantities. Frequently the gradual passing over 
may be observed from the normal gabbro into pyrrhotite 
gabbro, with from 5 to 95 per cent. of ore, and finally into 
the pure pyrrhotite. Very frequently, however, the con- 
tact between the ore and the gabbro is quite sharp, and the 
former is often observed to form, dykes or veins in the 
adjoining country-rock. Similar dykes are found composed 
of the intermediate product, the pyrrhotite gabbro, and the 
same class of phenomena @re also frequently observed in the 
case of differentiated deposits of titanic iron ores and 
chromic iron ores. Such occurrences may be explained by 
