122 NEW YORK STATE MUSEUM 



in rocks the same as at [Nlineville. and constitutes a reserve for the 

 future. 



It is natural to consider these last mentioned beds the northern 

 extension of the Smith mine, and it as representing the Old Bed 

 group, farther east and lower down than the Barton hill-Fisher 

 hill-Burt lot series : but inasmuch as the O'Xeill shaft is over a 

 mile from the last exposure of the Old Bed series with almost no 

 outcrops between, and in rocks that are practically massive, one 

 may quite as well regard the northern ones as totally distinct ore 

 bodies. Again one's train of thought is necessarily influenced by 

 the sedimentar}' or igneous views of origin. The axial trend of 

 the Smith mine is parallel to the same feature in all the others 

 to the south, and therefore shows the same great structural char- 

 acter, presumably due to folding, whose compressive strain being 

 at right angles to these axes, operated in a northwest, southeast 

 direction. 



Geological relations. Up to the time of the appearance of the 

 v.riter's paper on " The Geolog}' of the ^lagnetites near Port 

 Henry, N. Y., and Especially those at Mineville,'" in the Trans- 

 actions of the American Institute of Mining Engineers, 1898, 

 volume 2j, pages 146-204, the wall rocks of these great ore bodies 

 had been generally described as " gneiss," and had been with entire 

 justification regarded as the usual run of these ancient meta- 

 morphics which habitually contain the magnetites. By mapping of 

 each outcrop and parallel obsen'ations underground and by micro- 

 scopic determinations it was shown that there are several dis- 

 tinguishable types of gneiss present, and one intrusive gabbro. In 

 the '' Old Bed " group, of which the '" 21 " mine is the chief, the 

 hanging wall is a very light colored acidic variety which was called 

 the " 21 gneiss." It is a granitoid aggregate, consisting essentially 

 of microperthite and quartz. With these and in subordinate 

 amounts are plagioclase, magnetite, titanite and zircon. Of the 

 last named magnetite is the chief, and often the scattered grains 

 might easily give the observer the impression that they are some 

 dark silicate. Still, were a stray cr\^stal of the emerald-green 

 pyroxene also to appear, it would not be surprising, although one is 

 rare. An analysis of the rock is given on i>p. 49 and 50 and a 

 recasting for its mineralog}*. 



Additional study of many drill cores, and further observations 

 in the mines have served to corroborate the above determination. 



