1 66 NEW YORK STATE MUSEUM 



rocks rich in pyrite weather with deep rusty brown and black stains, 

 and very different rocks ma}^, for this reason, be regarded as identi- 

 cal in the field. While fresh specimens often show differences at a 

 glance, it is sometimes difficult to procure fresh specimens of such 

 easily weathered rocks. Without having made any extended study 

 of the sorcalied rusty gneisses, the writer has encountered two 

 rather distinct types. One of these contains much pyroxene, 

 hornblende, mica, feldspar, quartz, scapolite, a relatively large 

 amount of titanite, somxe zircon, graphite and both pyrite and 

 pyrrhotite. The minerals are often in rounded grains, and the 

 rocks might be classed as pyroxene granulites. Though occurring 

 in the Grenville, the origin of these rocks is obscure. Similar 

 though more massive rocks, with much scapolite, have, as already 

 stated, been classed as gabbros and were doubtfully placed in this 

 category by the writer. 



The rusty gneisses of the second type are of much simpler com- 

 position, having the ordinary minerals of acid gneisses together with 

 graphite and pyrite, with interlocking crystalline texture. From 

 the previous descriptions of the pyrite deposits it is obvious that 

 they all occur in gneisses of the latter type, being portions of the 

 rock exceptionally rich in pyrite. True, considerable variation of 

 composition appears in the gneisses as described, but not enough to 

 throw them into markedly distinct types nor any more than would 

 appear in descriptions of an equal number of specimens of the 

 ordinary Grenville gneisses. Perhaps the most pronounced 

 peculiarity of the pyritiferous gneisses, particularly as contrasted 

 with the pyroxenic rusty gneisses mentioned above, is their acidity, 

 they being always very quartzose and in general lacking the minerals 

 characteristic of basic gneisses. Even garnet and sillimanite, which 

 might be expected in these metamorphosed sediments, are by no 

 means common. 



While it is true that the pyrite ore bodies are confined to this type 

 of gneisses, it must not be forgotten that pegmatites present in. the 

 rocks also contain the mineral. The presence of these pegmatites is, 

 moreover, one of the striking features of the deposits, for at no 

 locality are they lacking. Whether or not they are more abundant in 

 connection with the ore deposits than throughout the region as a 

 whole, it would be difficult to say, but certainly they are no more 

 abundant than at many points where no pyrite occurs. The peg- 

 matite is often injected in thin layers parallel to the banding of the 

 gneiss and so thoroughly blended with the latter as to make it 



