30 NEW YORK STATE MUSEUM 



Essex county, notably around Crown Point. Compared with the 

 preceding group the most striking peculiarity of these magnetites 

 is the constant association of pyrite which brings the sulfur content 

 up to very considerable amounts, a feature that has been a serious 

 handicap to their development in the past. The pyrite may 

 possibly be traceable to original organic matter in the sandstones, 

 limestones and shales from which the present rocks have probably 

 been derived. The widespread occurrence of graphite in the same 

 rocks is noticeable. 



At Benson Mines, St Lawrence county, the ore body consists of 

 an impregnated zone in a quart zose banded gneiss. The gneiss con- 

 tains sillimanite and scapolite in addition to the feldspar, while the 

 dark minerals include hornblende, biotite and augite. Garnet and 

 pyrite are prominent. The walls in places are cut by a later horn- 

 blende granite. 



The Clifton mines, north of Benson, and those on Vrooman ridge, 

 near Fine, are found within a black hornblende schist with inter- 

 bedded layers of impure crystalline limestone. The latter occurs 

 next to the ore in one of the openings at Clifton. 



At Jayville the same sedimentary series is in evidence, though 

 here the ore bodies and walls (hornblende-biotite schist) have been 

 invaded by a great granite mass which has broken up what was 

 apparently a continuous bed into numerous lenses and shoots that 

 seem to give out in depth after passing the limits of the schist. 

 Curiously enough, the ore contains little pyrite. There is evidence 

 of recrystallization of the magnetite, and contact action has 

 caused the formation of great masses of hornblende and abundant 

 titanite. 



The several mines near Crown Point have opened on bands of 

 pyritous magnetite which are inclosed by a black hornblende gneiss 

 that has been correlated with the Grenville of this section. The 

 gneiss has been intruded by granite and in some places the latter 

 lies close to the ore. The ore bodies are parallel in all respects to 

 the St Lawrence county deposits. 



Origin of the magnetites 



The origin of these ore bodies has been variously interpreted by 

 geologists. The problem is an obscure one, involving as it does 

 accumulations of ores in rocks which are among the most ancient 

 known on the earth's surface and which in many cases have under- 

 gone great vicissitudes from compression and metamorphism. So 



