74 NEW YORK STATE MUSEUM 



that they seldom afford a basis for extensive mining operations. 

 The only occurrence of this character that has attained any economic 

 importance is the mine on Chilson hill near Ticonderoga, owned 

 by the American Graphite Co. The mine has been shut down for 

 the past 40 years. The deposit was not exhausted, but the great 

 depth attained together with the large influx of water prevented 

 profitable exploitation. The character of the pegmatite varies in 

 different places. Feldspar and quartz are the most common con- 

 stituents, while pyroxene, hornblende, mica, scapolite, calcite and 

 other minerals may be present. Owing to the coarse texture of the 

 materials the separation of the graphite ordinarily involves less 

 difficulty and expense than the usual type of deposit from which 

 the present output is obtained. 



Small veins of graphite of comparative purity are sometimes 

 found in the Adirondacks. The most notable occurrence is at Split 

 rock, south of Essex, Essex co., where there are several such veins 

 filling fissures in gneiss. The veinlets average less than an inch 

 in width and contain considerable quartz. They are not of suffi- 

 cient extent to repay working. 



The graphitic quartzites have proved so far to be the most valu- 

 able sources of the mineral in the Adirondack region. They repre- 

 sent ancient sedimentary rocks of the nature of sandstones which 

 have undergone metamorphism and recrystallization. They occur 

 principally on the borders of the region in scattered areas, which 

 are undoubtedly the remnants of a once extensive formation now 

 largely removed by erosion. Besides quartz, there is usually some 

 feldspar present, while the addition of mica may mark a transition 

 to mica schist. The graphite forms thin flakes or scales dis- 

 seminated through the rock. In quantity it ranges from a fraction 

 of I per cent up to 15 per cent, the usual run being less than 5 per 

 cent. The successful exploitation of the deposits depends upon 

 their large size and even tenor. The principal areas of graphitic 

 quartzite and schist occur in Warren, Washington and Essex coun- 

 ties in the vicinity of Lake George and Lake Champlain. They are 

 also known on the opposite side of the Adirondacks in St Lawrence 

 county. 



Crystalline limestones, a part of the Algonkian series of meta- 

 morphic rocks, are found in numerous places throughout the Adi- 

 rondack region. They underlie the valleys in long belts and occa- 

 sionally outcrop on the ridges. They were originally calcareous 

 sediments more or less charged with silica, alumina, magnesia, 

 organic matter, etc., and like the quartzites have received their 



