PETROLOGY IN AMERICA. 465 



itself. Vogt has recognised among iron-ores thus connected 

 with basic igneous rocks two chief types, one characterised 

 by titaniferous iron-oxides, the other by nickeliferous iron- 

 sulphides. Some of the former we have already referred to, 

 but examples of the latter are also known in America, the 

 case of Sudbury in Canada being cited by Vogt. Kemp (8) 

 has recently described an occurrence in Pennsylvania, in 

 which the relations are very clearly exhibited. The ore is 

 associated with a dark basic rock, now consisting mainly of 

 hornblende but probably an amphibolised norite or gabbro, 

 which forms a lenticular mass some 500 yards long. The 

 ore occurs as a marginal modification of this rock, and 

 consists of nickeliferous pyrrhotine and copper pyrites with 

 some iron pyrites and secondary millerite (nickel sulphide). 

 These minerals are associated with a certain amount of horn- 

 blende in such a manner as to show that they must have 

 been original constituents of the rock. 



The Adirondack gabbros afford some fine examples of 

 contact-metamorphism. The phenomena are most striking 

 at the contact of the gabbros with the crystalline limestones 

 of the district, and some well known-mineral localities fall 

 under this head. As described by Smyth (6), the lime- 

 stones become more coarsely crystalline as they approach 

 the gabbro, and pass finally into a zone consisting of various 

 lime-silicates and other special minerals, up to one or two 

 feet in width. In one occurrence this zone consists of 

 fibrous white wollastonite and grains of green pyroxene, 

 with some sphene and garnet. At other localities there are 

 several bands parallel to the junction, the one in contact 

 with the intrusive rock being of wollastonite, the next a 

 mixture of felspar, pyroxene, scapolite, sphene, zircon, etc., 

 and then coarsely crystalline calcite with much pyroxene. 

 Again, a layer of scapolite may occur in immediate contact 

 with the orabbro. Another mineral recorded is orthoclase in 



o 



well formed crystals, while phlogopite mica is generally distri- 

 buted. The author considers that there must have been 

 actual chemical reactions and interchange of material 

 between the intruded magma and the limestone. The 

 gabbro itself shows some modifications at the contact, the 



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