282 



EEPORT OF NATIONAL MUSEUM, 1899. 



They are fusible only with difficulty, and with the exception of the 

 mineral quartz are the hardest of the common light-colored minerals. 

 From quartz they are readily distinguished by their cleavage charac- 

 teristics noted above. Geologically the feldspars belong to the gneisses 

 and eruptive rocks of all ages, certain varieties being characteristic of 

 certain rocks and furnishing important data for schemes of rock classi- 

 fication. Nine principal varieties are recognized, which on crystallo- 

 graphic grounds are divided into two groups. The first, crystallizing 

 in the monoclinic system, including only the varieties orthoclase and 

 hyalophane; the second, crystallizing in the triclinic system, including 

 microclinic, anorthoclase, and the albite-anorthite series, albite, oligo- 

 clase. andesine, labradorite, and anorthite. The above-mentioned 

 properties are set forth in the accompanying table. 



Of the above those which most concern us here are the potash feld- 

 spars orthoclase and microcline, two varieties which for our purposes 

 are esssentially identical, ])oth as regards composition and general 

 physical properties as well as mode of occurrence. Indeed, although 

 crystallizing in different sj^stems they are as a rule indistinguishable but 

 by microscopic means or by careful crystallographic measurements. 



Occurrence. — The feldspars are common and abundant constituents 

 of the acid rocks — such as the granites, gneisses, syenites — the ortho- 

 clase and quartzose porphyries, and the tertiary and modern lavas — 

 such as trachyte, phonolite, and the liparites. 



Among the older rocks the}' not infrequently occur in large veins or 

 dike-like masses of coarse pegmatitic crystallization, the individual 

 crystals being in some cases a foot or more in diameter. The asso- 

 ciated minerals are quartz and white mica, with beryl, tourmaline, 

 garnet, and a great variety of rarer minerals. The ordinar}^ white 

 mi(^a of commerce comes from deposits of this nature and often the 

 two minerals are mined contemporaneously. Such of our feldspars as 

 have yet been worked for economic purposes occur associated only 

 with the older rocks — the granites and gneisses of the Archean and 

 Lower Paleozoic formations. 



Near Topsham, Maine, is one of these pegmatitic veins, running 



