32 NEW YORK STATE MUSEUM 



results. Of these the influence of highly heated vapors and waters 

 arising from the igneous mass has been most important. The occur- 

 rence of fluorite, apatite, hornblende etc., intercrystallized with 

 the magnetite, is suggestive in that line, as well as the frequent 

 accompaniments of pegmatite and vein quartz. This agency would 

 be specially active in the final stages of cooling and consolidation 

 of the wall rocks. In some cases it may have been the determina- 

 tive factor in bringing the iron minerals into their present position. 

 The ore bodies thus formed would be comparable in a way to 

 pegmatite dikes. 



Some authorities are inclined to doubt the efficacy of magmatic 

 differentiation as applied to the formation of ore bodies in rocks of 

 acid composition. There seems to be no valid reason for thus limit- 

 ing it to the gabbros and anorthosites of the Adirondacks. The 

 relative acidity of the rocks appears to the writer not so important 

 as the relation between the iron and lime-magnesia percentages. 

 With a large excess of iron over the amounts required for combina- 

 tion with the latter to form augite, hornblende and biotite, the 

 segregation of iron minerals might well be expected. This is exactly 

 the condition presented by the wall rocks of the ores. From the 

 analyses that have been given on a preceding page, it will be seen 

 that even N the more acid of the intrusives carry relatively high 

 percentages of free iron. The amounts of magnetite calculated for 

 the rocks, all of which are from localities outside of the mine dis- 

 tricts, run from 1.58 to 6.57 per cent. Higher percentages would be 

 found, undoubtedly, in specimens taken from the actual wall rocks. 

 With 5 or 6 per cent of magnetite a concentration of 10 to 1 would 

 produce the leaner ores that are mined in this region. 



The granites and syenites of the Adirondack iron ore districts 

 constitute a group that has some elements of relationship with 

 the gabbros and anorthosites. This is manifested by a similarity 

 in important features of chemical composition and by the existence 

 of transition types. The ores they inclose differ mainly in the 

 titanium content. In the silicious rocks, the titanium has com- 

 bined with lime and silica to form titanite which has been held 

 mainly in the body of the rock mass. With the basic magmas, 

 the silica has been entirely taken up by the feldspathic and ferro- 

 magnesian constituents and the titanium consequently united 

 with the iron and has been concentrated with it in the ore bodies. 

 The ores in the acid rocks commonly contain a fraction of one per 

 cent or so of titanium in the form of titanite. 



The pyritic ores that are found in the Grenville gneisses con- 

 stitute of course a distinct class. They may be ascribed possibly 



