46 NEW YORK STATE MUSEUM 



are not often very large. They appear to be more common across 

 the Hne in New Jersey than in the New York Highlands. One mass 

 just south of the town of Warwick can be traced for a long distance 

 but is very irregular, pinching in places to a thin film and again 

 bulging out into a body lOO feet or more across. There seems no 

 reason to regard the magnetite as other than an original constituent 

 of the pegmatite matrix and thus probably of deep-seated origin. 



A fourth type, illustrated by a single occurrence to the writer's 

 knowledge, is the Croton deposit in which the magnetite is included 

 within augite syenite. This occurrence has been described in con- 

 siderable detail by Koeberlein, and the present writer's observations 

 agree with his conclusions. The ore is a mixture of magnetite with 

 the minerals of the syenite which are mainly feldspar and pyroxene, 

 but contains besides more or less quartz, biotite, hornblende, apatite 

 and sulphides. A peculiarity of the ore in this section is the rim 

 of titanite which surrounds the magnetite grains. The magnetite 

 has crystallized after the silicates, in the reverse order of its con- 

 solidation in the igneous rocks which have low percentages of the 

 mineral — an anomaly that 'has been frequently noticed in the dis- 

 seminated ores of this type. The ore body grades off at the edges 

 into the normal syenite. If the wall rock is igneous, as is indicated 

 by its mineralogy and texture and also by its analogy with the 

 augite syenites of the Adirondacks, the ore is doubtless a magmatic 

 segregation in place. 



Another class of deposits differs from all the foregoing in that 

 the limestone forms one of the walls. The ore lies near the contact 

 also with granite or gneiss. The matrix consists of calcite and 

 silicate minerals like pyroxene, amphibole, garnet, chondrodi'te, 

 chlorite and serpentine. Spinel also may be present. The limestone 

 contact is usually marked by aggregates of the silicate minerals 

 which resemble the skam zones of some of the central Sweden 

 mines. The magnetite is rather fine in grain and on account of the 

 intricate admixture very hard and tough. Analyses show small 

 amounts of manganese. The shape of the ore body is more irregular 

 than that of the deposits in gneiss, only roughly approaching a 

 lenticular or tabular form. The silicate zones evidently are the 

 result of contact influence upon the limestone exerted by igneous 

 masses. The direct agent of metamorphism no doubt is granite or 

 its pegmatite derivative since the rock is always close at hand and 

 of the igneous representatives is the most el^cient in producing con- 

 tact alteration. The best known locality for magnetites of this 

 class is the Tilly Foster mine in Putnam county which has been 



