224 WATSON AND CLINE ROCKS OP THE BLUE RIDGE REGION 



nitely determined for all areas, but in several the evidence seems con- 

 clusive to the writers for regarding them as differentiation phases of the 

 same intrusive magma, since definite contacts between the two types have 

 nowhere been observed; but on the contrary there seems to be a gradual 

 passage from one into the other. In several of the areas on the southeast 

 slope of the middle Blue Ridge that have been studied in most detail it 

 is possible to collect specimens along a traverse that will range in the 

 amount of quartz from a minimum in the more typical syenite to a maxi- " 

 mum in normal granite, with practically all gradations between the two 

 extremes. It is regretted there are no available analyses of the more acid 

 type of rock of granitic composition. For such areas the field relation- 

 ships and microscopic study of thin sections are best explained on the 

 assumption that the granite is an acid extreme of the syenite. So far as 

 our studies have extended, we have not been able to note any border 

 phenomena in either rock of the several areas where the two are associated 

 which would indicate that one has been cut by the other. However, de- 

 tailed field studies have not been extended to all known areas of the rocks, 

 and to say that such relations do not exist would be unwarranted at the 

 present stage of our knowledge. In some places the granite occupies 

 areas up to several miles in width in which apparently no syenite appears, 

 while in other places a traverse will show alternations of granite and 

 syenite. The former relation is especially true of the granites mapped 

 and described by Keith 37 in the northern Blue Bidge of Virginia. 



The granites are even-granular, medium-grained rocks of fairly uni- 

 form texture, of light gray or pink color, and usually show indistinct to 

 pronounced foliation developed from pressure metamorphisni. In hand 

 specimens feldspar and quartz are easily distinguished, the latter being 

 of bluish color in some cases. Considerable green epidote is developed in 

 places, especially in the pink granite, yielding a rock similar in appear- 

 ance and composition to unakite (pages 220-223). Garnet is sparingly 

 developed and is largely altered to chlorite. 



Microscopic study of thin sections of the granites shows them to be, in 

 part at least, pyroxenic rocks. In most thin sections examined complete 

 alteration of the original ferromagnesian constituents to epidote, chlorite 

 or amphibole, and iron oxide rendered their determination impossible. 

 Cores of orthorhombic pyroxene rimmed by secondary fibrous amphibole 

 and iron oxide as alteration products were observed in a few thin sections 

 of the light gray granite. No trace of an original ferromagnesian min- 

 eral was noted in any thin section of the pink granite. "Whatever dark 



37 Arthur Keith : Fourteenth Ann. Rept. U. S. Geol. Survey, part ii, 1892-1893, pp. 

 285-395. 



