ADIRONDACK MAGNETIC IRON ORES 65 



As contrasted with the basic members acidic varieties are also 

 to be found most significantly in the cores but also in the natural 

 ledges. In the acidic varieties the dark silicates retreat, it may be 

 even to the vanishing point, while quartz enters and the rock 

 reaches well into the mineralogy of the granites. The feldspar is 

 most commonly microperthite as before and when mingled with 

 abundant quartz it yields the "21 gneiss" of the writer's earlier 

 article. Dark silicates almost entirely fail, plagioclase is rare, but 

 magnetite is invariably present in scattered grains, which in the 

 cores, unless care is taken, might readily be taken for a dark 

 silicate. This rock is one of great importance in the geology of the 

 ores since it is the common hanging wall of the Old Bed group. 

 Although in coarseness of crystallization it does not vary in texture 

 from the typical syenite, yet both in mineralogy and in rare asso- 

 ciated minerals it suggests the suspicion that it has affinities with 

 pegmatites, or that some influence such as the presence of vapors 

 or mineralizers aided in its development from the normal syenitic 

 magma. From this highly acidic and light colored rock the transi- 

 tion is abrupt to the dark basic masses of iron ore. 



Another light colored phase consists of oligoclase and quartz 

 with a few magnetites and zircons. This was specifically found 

 above the Barton Hill group, and was described in the writer's for- 

 mer paper as the " Orchard gneiss." It is after all a not very 

 different rock from the "21 gneiss." Microperthitic orthoclase 

 implies rich soda because of its albite spindles, whereas, if the 

 potash of the orthoclase fails and a slight increase of lime takes its 

 place, we have the necessary components of oligoclase. 



A still further variation from the normal augite syenite, is one in 

 which the feldspar and the dark silicates, augite and hornblende, 

 are in the usual proportions of say two thirds feldspar and one 

 third dark silicates, and yet oligoclase takes the place of the usual 

 microperthite. A rather acidic diorite results, but yet so involved 

 with the syenites as to prevent one drawing any distinctions 

 between them, as being separate intrusive masses. 



In the above condensed outline of the rocks, the characteristic 

 names of igneous types, syenite, gabbro, diorite etc., have been 

 employed, implying that the rocks themselves are igneous. This 

 is opposed to the older idea which is generally still held by the 

 engineers and others engaged in the mining industry. The latter 

 view the ores and their inclosing rocks as sediments, which conform 

 in a pronounced degree to the sedimentary structures shown by 

 strata in parallel arrangement and in folds and faults. The writer 



