ADIRONDACK MAGNETIC IRON ORES 2.*] 



There can be no doubt that the form assumed by the ore bodies 

 is conditioned by the structures of the inclosing rocks. When the 

 latter are foliated to an extent that permits observations of dip 

 and strike, the contours follow the changes closely, even the sub- 

 ordinate ones. This feature is least apparent in the gneisses of the 

 igneous series, the structures of which are often only faintly indi- 

 cated, and most evident in the banded gneisses and schists of the 

 Grenville. The ores consequently must have been deposited 

 before the great regional disturbances took place, or at least before 

 the rocks received their present structural arrangement. They 

 have passed through all the vicissitudes of squeezing, folding and 

 other deformations that have been impressed upon their walls. 



In their original condition the ore bodies were probably tabular 

 masses, like those now existing in the regions of least disturbance. 

 From such masses, a complete sequence may be traced into lenses, 

 shoots and the more complicated structures that have been devel- 

 oped by operation of mechanical processes. 



Associated rocks 



There is no constant type or formation that is characteristic for 

 the nontitaniferous ores as a whole. The wall rocks include gneisses 

 of granitic, syenitic and dioritic composition, acid pyritic, gar- 

 netiferous gneisses, hornblende and biotite schists, amphibolites 

 and occasionally crystalline limestones. 



From considerations of their probable origin, they may be 

 divided into (i) igneous derivatives and those closely allied to the 

 characteristic intrusive masses of the Adirondacks; (2) members 

 of the sedimentary or Grenville group. Nearly all of the magnetite- 

 bearing rocks may be referred with a degree of certainty to the one 

 or the other of the two classes. For a few occurrences, however, 

 the evidences of relationships that have been found thus far are too 

 obscure to admit any definite conclusions, though it is probable 

 that the rocks are uniform with the others, rather than character- 

 istic of a distinct class. 



1 Igneous group. The more acid members of the igneous series 

 constitute the country of the Clinton county mines, all of which 

 occur within the belt of alkali-feldspar gneisses known as the Saranac 

 formation. At Lyon Mountain, the country consists of a massive 

 reddish variety composed of microperthite, oligoclase, green augite, 

 hornblende and magnetite with a small amount of quartz. Mineral- 

 ogically it lies on the border between the syenite and granite rock 



