ADIRONDACK MAGNETIC IRON ORES 45 



undertaken by the Oliver Iron Mining Co., for the purpose of 

 establishing a basis for exploratory operations with the diamond 

 drill. The field work was carried on during the summer of 1906 

 under charge of Mr M. H. Newman, who has afforded the writer 

 every opportunity to keep in touch with its progress and likewise 

 to make use of the results. The district is extremely complex 

 geologically by reason of the great variety of rock formations 

 represented, which involve practically the whole series of Adiron- 

 dack crystallines, and the intricate structural relations resulting 

 from plication, faulting and the intrusion of igneous masses. 



The formations may be divided in a general way into the Ham- 

 mond ville or ore-bearing gneiss, which is a quartz-plagioclase 

 gneiss of doubtful relationships; a group of metamorphosed sedi- 

 ments including crystalline limestone and hornblendic and mica- 

 ceous gneisses and schists, and an igneous series composed of 

 anorthosite, gabbro, diabase, syenite and granite. This is essen- 

 tially the classification proposed by Dr Ogilvie except that the 

 Hammondville type of gneiss is considered by her to be eruptive 

 and is mapped with the granites. 



Hammondville gneiss. The rock inclosing the deposits is dis- 

 tinguished by a finely granular cataclastic texture and almost 

 entire absence of dark minerals except magnetite. It has a homo- 

 geneous character for the most part, in contrast with the recog- 

 nizable members of the sedimentary gneisses which vary greatly 

 from place to place. Of the igneous rocks exposed in the district, 

 it most closely resembles the granite, but differs in some particu- 

 lars of mineral composition and in the more intense crushing 

 effects which it exhibits. 



Mineralogically it consists almost wholly of plagioclase feldspar 

 and quartz. The ferromagnesian constituents are limited to 

 occasional shreds of biotite and a little green hornblende, forming 

 an inconsiderable proportion of the mass. Magnetite is fairly 

 abundant, in many specimens richly so. Apatite, titanite and 

 zircon are the remaining components. 



The rock has uniformly a grayish color on unweathered surfaces, 

 changing to brown in exposures, with sometimes a reddish stain 

 from a little included pyrite. In the finely crushed phases it looks 

 much like a feldspathic quart zite. In the percentage of silica 

 present it corresponds to an acid granite with an indicated content 

 of 70 per cent or more, but it differs from usual granites in the pre- 

 dominance of the soda feldspar, the potash varieties being practi- 

 cally absent. The magnesia and iron are below the average for 



