THE ORIGIN OF THE PHENOCRYSTS 1 15 



Microcline and orthoclase occur porphyritically developed, 

 and measure 15 to 50 mm in length. The phenocrysts possessing 

 idiomorphism usually display the Carlsbad habit of twinning, 

 and are elongated in the clinopinacoidal direction. 



A thin section of one of the phenocrysts showed the charac- 

 teristic microcline structure and numerous inclusions of quartz, 

 biotite, and the groundmass feldspars, which measure as much 

 as one millimeter in size. Prismatic crystals of apatite and zir- 

 con, as inclusions in both the phenocryst and the included ground- 

 mass feldspars of the porphyritic crystal, are numerous. The 

 zircon crystals are sometimes grouped in threes, much after the 

 manner of penetration twins. 



RESUME 



Since the individual areas have been described in some 

 detail, it is important that a general summary of the essential 

 features common to the several porphyritic granite masses be 

 given. These can best be summarized under the two headings, 

 macro- and microscopic features. 



Macroscopic features. — The same textural and structural char- 

 acteristics and relationships are generally developed in all the 

 individual porphyritic granite areas. With the exception of the 

 Warren (W-H) county area, the rocks are prevailingly massive, 

 coarse-grained, porphyritic granites, varying, according to the 

 proportion of biotite present, from dark to medium gray in 

 color. The Warren county granite differs structurally from that 

 of the other areas in possessing a marked foliated structure, 

 resulting from dynamic metamorphism, which is accordingly 

 secondary. Further evidence already mentioned of dynamic 

 action in this rock mass is apparent microscopically. Evidence 

 of a partial gneissoid structure is indicated in portions of several 

 of the other areas, but, as a rule, the rock is generally massive. 



Field study shows the development of the porphyritic tex- 

 ture in the interior of the rock masses, with the even-granular 

 granitic facies having the same mineral and chemical composi- 

 tion forming the marginal or body portion. Gradation from 



