ADIRONDACK MAGNETIC IRON ORES 13 



included impurities by regional and contact metamorphism. 

 Pyroxene, amphibole, mica, graphite, pyrite and scapolite are 

 common associates. With an increase in the proportion of the 

 silicate minerals, the limestones pass into micaceous, pyroxenic 

 or amphibole schists. By secondary alteration of the pyroxene a 

 serpentinous limestone or more rarely a massive serpentine may be 

 developed. 



The limestones and associated schists are found generally in long 

 narrow belts bordered by the sedimentary gneisses. They are 

 most widespread on the northwestern side of the Adirondacks in 

 St Lawrence, Jefferson and Lewis counties. Four main belts, with 

 a length of from 15 to 35 miles, and a great number of smaller ones 

 have been mapped in this region. On the east side they occur most 

 abundantly in Essex county, but they are here less extensive. In 

 the interior and on the northern and southern borders, the lime- 

 stones are encountered in disconnected patches, occasionally 

 interfolded with the igneous rocks in which they were, no doubt, 

 involved during the intrusion. 



Gneiss. The sedimentary gneisses are an extremely varied 

 class. Their many phases comprise light colored acid types made 

 up purely of quartz and feldspar, gray or dark gneisses in which 

 the ferromagnesian minerals are represented more or less abund- 

 antly, and black basic varieties with only subordinate feldspar or 

 quartz. Wide differences in composition are often observable 

 within the limits of a single outcrop, particularly in passing across 

 the foliation. The transitions from one variety to another take 

 place quickly and lend the appearance of a banded arrangement 

 comparable to that of bedding among unaltered sediments. Still 

 there are districts in which the gneisses show a fair degree of uni- 

 formity, and their relations are only to be established after careful 

 investigation in the field and laboratory. The presence of graphite 

 is common and suggestive. Garnet, sillimanite and pyrite are also 

 characteristic minerals. Where pyrite occurs the beds weather 

 rapidly, taking on a peculiar rusty appearance. The texture of 

 the gneisses is always granular, as a rule finely so, due to the 

 intense crushing they have undergone. 



The distribution of the sedimentary gneisses corresponds in a 

 general way to that of the limestones, being most widely developed 

 on the borders of the region. They occur, however, over consid- 

 erable areas where limestone may be relatively scarce. In the 

 northern Adirondacks, Cushing has found them to be of small 

 importance, as the main formations are igneous or of so question- 



