Ball — Pre- Cambrian Rocks of Georgetown, Col. 385 



Pegmatite. — While each, of the granular igneous rocks 

 already described grades into a pegmatite, the pegmatite of 

 most interest is that which intrudes or grades into the youngest 

 pre-Cambrian granites and granite-porphyry. In a single 

 exposure the pegmatite and granite may grade into one 

 another at one point and at another the rocks may be sharply 

 differentiated, the pegmatite as a rule being younger. 



Salmon-pink feldspar, either orthoclase or microperthitic 

 microcline, is usually the predominant mineral of the peg- 

 matite. A greasj^-gray, acid plagioclase is restricted in its dis- 

 tribution to pegmatite dikes in the quartz-monzonite. Slightly 

 smoky quartz, which microscopic examination shows to con- 

 tain abundant opaque thread-like interpositions and myriad 

 fluid inclusions, is always present and, in the tiny banded 

 quartz apophyses which are given off from the large dikes, it 

 is the only constituent of the pegmatite. Biotite is more com- 

 mon than muscovite and the two micas characteristically occur 

 in separate bodies of pegmatite, although they are sometimes 

 associated in the same masses and in rare instances a core of 

 biotite is surrounded by a muscovite border. An interesting 

 form of pegmatite is composed of diamond-shaped muscovite 

 plates up to 1 inch in diameter embedded in quartz, each 

 mineral being present in nearly equal amounts. The rock is 

 closely allied to beresite from the Ural mountains and to a 

 rock described by Mr. J. E. Spurr from Belmont, Nevada.* 



Magnetite is a widely distributed constituent and in some 

 cases forms over one-third of the pegmatite mass, which in 

 consequence becomes a lean iron ore. Magnetite occurs in 

 octahedral crystals of a maximum diameter of 4 inches or in 

 irregular aggregates up to 6 inches across. It is in some 

 instances the only femic mineral of the pegmatite, but is 

 usually associated with biotite and less often with muscovite. 



Black tourmaline is a widely distributed but never abund- 

 ant mineral in the pegmatites. It occurs either in crystals 

 embedded in quartz or orthoclase, in micropegmatitic inter- 

 growths with quartz and feldspar, in feldspar metasomatically 

 replacing it or in felts along cracks in the pegmatite. Hence 

 while tourmaline usually solidified prior to quartz and feldspar, 

 it sometimes solidified contemporaneously with them and 

 rarely after them. The femic mineral usually associated with 

 tourmaline is muscovite, both of which minerals from other 

 localities usually contain some fluorine. Red garnets are 

 locally very abundant in the more siliceous pegmatites. Bare 

 constituents include allanite, apatite, hornblende, beryl and a 

 quartz -feldspar pseudomorph, probably of spodumene. 



* This Journal, vol. x (1900), p. 351. 



