164 NEW YORK STATE MUSEUM 



Throughout the region covered b}' these intrusives are de- 

 posits of aluminous titaniferous magnetite. On the eastern and 

 southeastern borders this ore becomes very aluminous with a 

 proportionate decrease in the amount of iron, and in these por- 

 tions it is mined for abrasive purposes. 



Prof. G. H. Williams, in a paper on the norites of the " Cort- 

 landt series/'^ published the results of a very careful study of 

 these deposits. It was his opinion, after extended microscopic 

 inspection of this material, that it consisted chiefly of am iron- 

 magnesian spinel (hercynite), and that there was a striking 

 similarity between these ores and certain magnetite deposits at 

 Koutivara, Sweden. Prof. J. F. Kemp has remarked a like simi- 

 larity between these deposits and certain of the emery deposits 

 in North Carolina. The latter are not, however, titaniferous to 

 an}- marked extent.^ 



The deposits according to Professor Williams are segrega- 

 tions of the basic minerals of the norite, the purest of the emery 

 being found to contain all the component minerals of the norite.^ 



An inspection of a series of thin sections of material from 

 these deposits^ under the microscope showed that they consisted 

 of hercynite, magnetite and corundum of a very light color. Of 

 these minerals the hercynite was by far the most abundant, 

 forming in some cases over 50^ of the material in the slide. 

 The corundum occurs in small crystals containing medial in- 

 clusions of what appeared to be magnetite. Magnetite in grains 

 showing a crystalline outline is also included in the hercynite. 



The proportions of corundum and hercynite are very variable. 

 In some specimens the corundum will make up over 50^ of the 

 slide, while in others the material is almost 100^ hercynite. 



The hercynite is inferior in hardness to the corundum, 

 corundum being 9 in the scale of hardness, while hercynite is 



^Am. Jour. Sci. Ser. 3. 1887. 33:194. 



'Kemp, J. F. School of Mines Quarterly. July 1899. p. 345. 

 "Am. Jour. Sci. Ser. 3. 33:196. 



*A series of slides of this material was furnished the author by the 

 kindness of Prof. J. F. Kemp, of Columbia University. 



