MINERAL RESOURCES OF THE STATE OF NEW YORK 1 5 



converted the limestones into masses of secondary silicates. The 

 last phase of igneous activity is represented by the intrusion of 

 diabase in dikes which penetrate all the other Precambrian rocks 

 and must have occurred late in that era. 



The mineral deposits of the Adirondacks are associated with 

 both the igneous and the Grenville sedimentary rocks. Of those 

 having an igneous environment the iron ores are most extensively 

 developed and commercially important. They consist of magnetites 

 in bands, lenses and irregular bodies which have granite and syenite 

 for wall rocks. Another class is the magnetite-ilmenite mixtures 

 in the basic igneous materials — anorthosite and gabbro. The 

 principal deposits of the latter are in the interior of the great 

 bathylith, near the Mt Marcy district of Essex county, where there 

 are very large bodies of the ores. The magnetites free of iknenite 

 are distributed among the smaller bathyliths and stocks of the 

 granite-syenite series in the eastern, northern and northwestern 

 Adirondacks, mainly within the limits of Essex, Clinton and St 

 Lawrence counties. The southern and southwestern regions have 

 no considerable deposits. Among other minerals carried by the 

 igneous rocks are apatite which is foimd in quantity in admixture 

 with the magnetites of Mineville, fluorite also associated with certain 

 of the magnetites, molybdenite in sporadic occurrences in granite 

 of uncertain economic importance, and feldspar, quartz and mica, 

 ingredients of the pegmatitic phases of the granites. All of the 

 materials mentioned are genetically related to the different igneous 

 rocks in whose environment they occur, although there is some 

 doubt about the particular method by which certain deposits have 

 been accumulated. The magnetite-ilmenite mixtures are in the 

 nature of segregations of the gabbro and anorthosite magma, effected 

 doubtless by gravity during the period of crystallization. The 

 same principle may have been operative in the formation of the 

 magnetites of the granite and syenite rocks, although in some 

 examples the influence of mineralizing agencies is indicated by the 

 presence of such minerals as fluorite and apatite while the greater 

 mobility of the magmas has permitted the migration of the ores 

 from their original environment. 



The Grenville strata contain a considerable array of ores and 

 mineral materials. Compared with the igneous formations they 

 are more permeable to underground water circulations, having more 

 open spaces and in the case of the limestone especially being con- 

 siderably more soluble. They are consequently the country rocks 

 for most of the ores assembled as veins and replacements, by the 



