ADIRONDACK MAGNETIC IRON ORES 149 



-differing in composition but of the same genesis as the anorthosite 

 and gabbro. The magnetite and ilmenite of which they are aggre- 

 gates exist in the country rocks as accessory constituents. A 

 concentration that took place probably during the cooling of the 

 magma effected the segregation of the heavy minerals into com- 

 pact masses forming ore bodies of variable size and richness. This 

 view of the relation of the Adirondack ores has been clearly brought 

 ■out by Professor Kemp x in the following terms : 



In the preceding pages the point of view has been consistently 

 maintained that the ore bodies are integral portions of the igneous 

 rocks in which they occur and are merely local enrichments of 

 the mass with unusual amounts of one of its normal constituent 

 minerals. This has not been done with the purpose of advocating 

 one conception of the relations of the ore and wall rock to the 

 ■exclusion of others, but because the observed phenomena admit 

 of no other reasonable interpretation. There is no evidence of 

 the replacement of preexisting material by an entering foreign sub- 

 stance, nor of faults and vein formation, nor of crushed zones 

 different from the neighboring walls ; nor are the ores at the con- 

 tacts of intrusions with country rock. On the contrary, the masses 

 of ore, of irregular shape, are far within the intrusions, and especi- 

 ally in the gabbros they vary from rich titaniferous iron oxide, 

 through leaner and leaner examples, until normal gabbro is reached. 

 No minerals or elements occur in notable amounts in the ores which 

 are not characteristic components of the wall rock. The difference 

 between ore and rock is one of degree and not of kind. At 

 Calamity brook the ore itself forms a series of dikes in country 

 rock of a different kind. 



The causes acting to produce such a concentration or magmatic 

 differentiation are little understood. Gravity, convection cur- 

 rents, magnetism, and diffusion consequent upon variation in the 

 rate of cooling are some of the agencies that have been appealed 

 to by the leading investigators to account for the accumulation 

 of the deposits. 



It is of interest to note that the igneous theory of derivation 

 for these ores which has come into prominence in recent years 

 and is now generally accepted by geologists the world over, was 

 foreshadowed by Professor Emmons in his report on the Adiron- 

 dack region for 1842. The ore occurrences at Lake Sanford were 

 designated by him as " masses," to distinguish them from the 

 " veins " or tabular bodies occurring in the gneisses, and they 

 are described as of contemporaneous origin with the inclosing 

 rocks which he recognized to be igneous. 



1 Titaniferous Iron Ores of the Adirondacks, p. 417. 



