150 NEW YORK STATE MUSEUM 



Shape of the ore bodies 



The form assumed by the ore bodies is not always apparent 

 from the field evidence. It is only the smaller ones as a rule that 

 are well exposed in outcrop. The large bodies have nowhere been 

 uncovered or explored sufficiently to afford an idea as to their 

 precise outlines. The smaller bodies, with a few exceptions found 

 at Lake Sanford, occur in gabbro which generally appears in. 

 dikes cutting the anorthosite, and partake of the usual tabular 

 form with the longer axis parallel to the strike of the dikes. They 

 show gradation at the edges into the normal gabbro and their 

 materials have no doubt come up with it from a common reser- 

 voir below. As to the deposits inclosed by the anorthosite, it is 

 not conclusive whether the ores have separated in place from the 

 surrounding rock, or whether they represent later concentrations 

 in the interior that have been intruded into the anorthosite after* 

 its partial solidification. In the former case we should expect 

 the bodies to be quite irregular, with no well defined walls, and 

 to shade off at the borders with a gradual increase in the propor- 

 tions of gangue material or rock. From the evidence at hand, 

 the large bodies like the Lake Sanford would appear to be allied 

 rather to that type than to the dike form. 



Mineralogy of the magnetites 



The titaniferous ores of the Adirondacks are essentially aggre- 

 gates of magnetite and ilmenite. The richest ores contain little else 

 than these minerals and show on analysis 60 per cent or slightly more 

 of iron, the maximum percentage being somewhat below that of 

 the high-grade nontitaniferous magnetites. From such pure aggre- 

 gates there may be traced a continuous series of gradations, by the 

 entering of gangue minerals in greater and greater proportions, to 

 the limiting wall rocks which hold only subordinate amount 

 magnetite and ilmenite. 



The relations of the two iron minerals have received, hitherto, 

 little attention. The presence of ilmenite has been inferred from the 

 results of chemical analyses; its identification by the usual optical 

 methods of petrography is difficult owing to its opacity and simi- 

 larity of appearance to the magnetite with which it is intimately 

 associated. 



Ilmenite is not uniform in its composition and its chemical 

 nature has been the source of considerable perplexity to mineral- 

 ogists. The view that it is a metatitanate Fe Ti0 3 has the support 



