INTEBMEDIATE CHARACTER OF APPALACHIAN GRANITES 3 



(2) MetamoriDhism of the rocks, chiefly reerystallization and schistose 

 structure, is strongly emphasized in the gneisses deri\-ed from original 

 massive-granular, acid, intrusive granites. In the massive rocks the 

 most pronounced effects are seen under the microscope in the optical 

 disturbance of some of the minerals, chiefly quartz and to a less extent 

 feldspar. Partial peripheral granulation of these two minerals and 

 irregular fractures are sometimes shown. As a rule, the massive acid rocks 

 do not grade into the more pronounced schistose forms, granite-gneisses, but 

 the two are usually sharply defined from each other. With only few excep- 

 tions, the acid gneisses studied are derived from original granites by nieta- 

 morphism, and have been designated granite-gneisses. The porphyritic and 

 even-granular acid irruptives grade into each other and are regarded as 

 textural variations of the same rock-mass. Potassic feldspar, orthoclase or 

 microcline, is the porpliyritically developed mineral in the porphyritic facies 

 of the rocks. The phenocrysts vary in size, may be allotriomorphic or idio- 

 morphic in outline, show good cleavage development, usually without 

 orientation indicated, and contain inclusions of the principal groundmass 

 minerals, especially biotite, and are regarded as having been formed in 

 place. 



(3) Tlie acid-granular rocks are not of the same age but have been 

 intruded during several diiferent geologic periods. Their exact age is 

 uncertain, but a part of them (granite-gneisses) are pre-Cambrian, while 

 the massive granites are of early or later Paleozoic age. 



(4) Closely similar mineral and chemical composition characterize these 

 rocks throughout the entire region. The close similarity in mineral and 

 chemical composition of the rocks is emphasized in subsequent pages of 

 this paper, and need not be repeated here. 



(5) Similar textural, structural, and color variations in the rocks are 

 noted but not with the same frequency over all parts of the region. 

 Texturally, variation is from fine- to coarse-grained even-granular to por- 

 phyritic rocks, and structurally from massive to completely schistose. Some 

 shade of gray (light, intermediate, or dark) is the usual color, with pink 

 not uncommon in places. 



(6) Characteristic contact metamorphic zones frequently observed alDout 

 the border positions of large granite intrusions are seldom observed. 

 Apophj^ses or dikes of the same composition as the parent rock frequently 

 project outward from the main granite-masses and cut abruptly into the 

 adjacent rocks. Pegmatites are also found penetrating, as oflishoots, the 

 rock masses enveloping the stocks of granite. Incorporated fragments of 



