66 NEW YORK STATE MUSEUM 



jointed and fractured condition. Serpentine is one of the softer 

 minerals and on that account the rock can not be applied to general 

 constructional purposes, but finds a market chiefly as an ornamental 

 material by reason of its lustrous green color and of the striking 

 pattern produced by the blotches or veinings of iron ores and 

 calcite. 



Besides this kind of serpentine, mention may be made of serpen- 

 tinous marbles or ophicalcites which are derived from impure 

 sedimentary limestones. In the metamorphism of the limestones, 

 pyroxene is formed which later changes over to serpentine, giving 

 a mottled or spotted effect of green on a white body of calcite. 

 Such serpentinous marbles occur in the eastern Adirondacks and 

 have been used to a limited extent for monumental and interior 

 decorative work. 



PEGMATITE 



Pegmatite is really a member of the granite series, being a coarse- 

 grained intrusive composed of feldspar, quartz and mica. It has 

 little value for structural purposes which granite serves, and in its 

 mode of occurrence and origin differs somewhat from the ordinary 

 representatives of that series. It is found in dikes with fairly 

 regular tabular form, but also occurs in irregular winding veins 

 and occasionally in masses that show a lenticular or rounded out- 

 crop like bosses of the finer grained igneous rocks. The latter 

 type may attain very large proportions, that is a thousand feet or 

 more in diameter, while the dikes seldom exceed 40 or 50 feet in 

 thickness and for the most part are under 10 feet. 



The mineralogy of pegmatites is of much interest on account of 

 the variety and fine crystallizations of the species that accompany 

 them. The important species, however, are the same as those 

 described as essential constituents of granite. The quartz is com- 

 monly .white, gray or pink, occurring in crystals or massive, and 

 ranging from a few inches to several feet in diameter. It is also 

 more or less intergrown with the feldspar, sometimes in a peculiar 

 way which is known as " graphic granite." The feldspar includes 

 the alkali varieties like microcline, orthoclase and albite, with 

 usually more or less of lime-soda feldspar of oligoclase or andesine 

 nature. Individual crystals sometimes measure 5 or 6 feet long. 

 Both the quartz and feldspar are valuable where they can be ob- 

 tained in condition of fair purity and uncontaminated by iron ; their 

 principal use is in pottery, but they serve many other purposes. 

 The mica of pegmatite belongs to both the lighter iron-free sorts 

 like muscovite and phlogopite and the dark variety biotite ; it builds 



