220 The American Geologist. -^p'"- ^^^i- 
they do not require extended details further than given under 
the descriptions of the various granite-types. There are sev- 
eral minerals present, however, which from their association 
and conditions of environment require further descriptions. 
These are microcline and muscovitc. 
The prevalence of intergrowths with a second feldspar 
common to the three rock phases, in case of orthoclase and to 
some degree microcline, in the form of microperthitic struct- 
ures, has already been remarked on under the descriptions of 
the individual types. 
Microcline : The abundance of feldspar grains showing 
the characteristics cross-grating structure is a striking feature 
in most thin sections of the Georgia granites. The twinning 
lamellae thin out in many cases, and do not always show per- 
fectly parallel sides. Crystals are often met in which the 
microcline structure is developed in only a part of the individu- 
al and entirely failing in the other parts, leaving the untwinned 
area optically indistinguishable from orthoclase. These granites 
have been subjected, as elsewhere shown, to intense dynamo- 
metamorphism, which fact, coupled with the prevalence of this 
feldspar, affords strong indications of a part of this mineral 
having acquired its structure. A part of the microcline is un- 
questionably primary. Similar conditions are recorded in 
granite masses in different localities by various writers.* 
Muscovite : Since the occurrence of primary muscovite 
in an eruptive granite has been questioned, it is of interest to 
note a similar occurrence of this constituent in many of the 
Georgia granites to that observed by Keyesf in some of the 
Maryland granites, where primary origin affords the best ex- 
planation. In many of the Georgia granite areas, especially 
the Oglesby-Lexington dark blue granite area, large and 
sharply bounded plates of fresh appearing muscovite, are dis- 
tributed through the rock in intimate association with the bio- 
tite. The two micas are frequently grouped together as aggre- 
gates in which the individuality of the separate crystals is well 
preserved. They not infrequently form parallel intergrowths, 
• Keybs, C. K., 15th Ann' I Kept., V. S. G. S., 1893-94, pp. 711-712. 
Von Rinnk, Neues Jaiirb, ii, 1890, pp. 66-70. ] 
Bkutellk, Zeitsciir. t". Krvst, viii, p. 373. I -^„„4.„,i ,,„ t-„„„„ 
Sauer & UssiNG, Ibid, xvifi, 1891. p. 196. ( Q"oted by Kcyes. 
Brogger, Ibid, xvi, 1890, p. 561. J 
t Keyes, C. R.. loth Ann'I Kept., U. S. G. S., 1893-9-i, pp. 703-704-. 
