PETROGRAPHY OF THE GRANITES 527 



little muscovite, and it consists of anhedra which range in size from 

 1 to 5 millimeters. Variation in the size of anhedra here given is a 

 noticeable feature of this type in the quarries opened around the cities 

 of Eichmond and Petersburg. The principal minerals are quartz, ortho- 

 clase, microcline, plagioclase near oligoclase, biotite, a little muscovite, 

 sphene, magnetite, apatite, and zircon. The usual alteration minerals 

 occur, chief among which are chlorite, epidote, muscovite, and kaolin, 

 These are the normal minerals in granite. 



Eutile needles are abundant in some of the quartz. Granophyric inter- 

 growths of the quartz and feldspars are very abundant in the thin- 

 sections studied, indicating overlap in the period of formation of these 

 minerals. The quartz intergrowth is not restricted to any single species 

 of feldspar, but it seems to be of about equal development in the ortho- 

 clase, microcline, and plagioclase. Orthoclase is usually the dominant 

 feldspar, but microcline or plagioclase may either equal or even exceed it 

 in amount. An important feature in the mineral composition of these 

 rocks is the nearly constant large amount of plagioclase present. It 

 occurs in large stout laths and is always characterized by the polysyn- 

 thetic twinning striae in basal sections. Extinction angles measured 

 against the. twinning striae usually indicate a plagioclase near oligoclase. 

 Twinning on the Carlsbad law is very common among the feldspars, both 

 in thin-sections and in hand specimens of the rock. Microperthitic inter- 

 growths of the feldspars are freely developed in most of the thin-sections 

 studied. Many of the larger feldspars are micropoikilitic, the inclosures 

 consisting of quartz and feldspars. 



Biotite is the third essential constituent ; it is deep brown and strongly 

 pleochroic. It is distributed through the rock in single long and stout 

 shreds and as small aggregates. This constituent is subject to some 

 variation, both in size of shred and in amount, the rock becoming, accord- 

 ing to quantity, either lighter or darker in color. Under the microscope, 

 numerous inclusions of the older minerals are seen in the biotite, among 

 the most important of which are zircon, apatite, and magnetite. In a 

 majority of the thin-sections a little primary muscovite is intimately 

 associated with the biotite, occasionally as parallel growths, and, at 

 times, the two micas penetrate each other, always preserving sharp and 

 clear-cut boundaries. Much chlorite occurs as an alteration product of 

 the biotite. Epidote, while not abundant, is usually present in many of 

 the sections, always as an alteration product from the interaction of the 

 biotite and feldspars, and is associated with the biotite. 



Titanite is present to a limited extent in many of the sections, mostly 

 as irregular grains and aggregates, but sometimes as wedge-shaped crys- 



