QUARRY MATERIALS OF NEW YORK 67 



sheets and thicker plates that attain a size up to 2 or 3 feet across. 

 Its occurrence in pegmatite is the source of commercial mica, but 

 the mineral has to be free of inclusions and checks to be of much 

 value, which is very rarely the case in any of the Adirondack 

 pegmatites. 



In addition to quartz, feldspar and mica, there are a great many 

 minerals that occur in more or less abundance in the local pegma- 

 tites. Some of the commoner ones are tourmaline, beryl, garnet, 

 amphibole, magnetite, pyrite, apatite, zircon, titanite, lepidolite, 

 chlorite, epidote and calcite ; of rare occurrence are monazite, xeno- 

 time, autunite, dumortierite, molybdenite and allanite. The crys- 

 tals of tourmaline and beryl may weigh many pounds. 



Pegmatites are quite variable in their composition, changing 

 much more rapidly in that respect than granite. The proportions 

 of feldspar and quartz fluctuate through all possible ranges, as may 

 be seen in almost any of the larger bodies like those at Crown 

 Point and Bedford, for example. A mass of practically solid feld- 

 spar in one place gives way in a short distance to one of quartz or 

 to a mixture of the two minerals. These fluctuations take place 

 horizontally and vertically and often are the cause of much incon- 

 venience if they do not seriously affect the progress of quarrying, 

 especially where it is aimed to secure a uniformity of products. 

 In many quarries this feature seems to have been ignored at first, 

 and the results of work consequently have not corresponded to 

 expectations. There is need of careful investigation to determine 

 the character and uniformity of the materials in each locality which 

 should precede actual development. Bosses and large dikes of 

 pegmatite extend downward into the earth for indefinite distances, 

 usually much farther than they can be followed in open quarry 

 operations. The lenses and veins are much less persistent, often 

 pinching out abruptly. 



Pegmatite is associated with many of the granite areas in the 

 Adirondacks and southeastern New York. In most of the granite 

 quarries small irregular masses of the material are encountered, in 

 some with such frequency as to impair the value of the product. In 

 the larger occurrences the pegmatite may be left as a wall in the 

 quarry. The irregular bodies which grade over into the granites 

 are apparently not intrusive in the latter, but have resulted from 

 crystallization of .the magma in place, the coarse texture being due 

 to the local presence of abundant water vapor and other mineraliz- 

 ing agencies. The pegmatite is probably the last part of the mass 

 to crystallize and represents the residue of magmatic material with 



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