QUARTZ-BEARING HYPERSTHENE-ANDESINE SYENITE 



203 



II. Hypersthene syenite near the southeast foot of Tobacco Row Mountain, 

 in the vicinity of Elon, Amherst County, Virginia. J. G. Dinwiddie, 

 analyst. 



III. Hypersthene syenite from the northwest slope of the Blue Ridge near 

 the boundary of Madison and Greene counties, Virginia. J. G. Din- 

 widdie, analyst. 



IV. Hypersthene syenite from the base of the northeast slope of the Peaks of 

 Otter, Bedford County, Virginia. J. G. Dinwiddie, analyst. 

 V. Hypersthene akerite (syenite) from Milams Gap, Page and Madison 

 counties, Virginia. W. C. Phalen, analyst. 



Norms corresponding to Analyses of Rocks on page 202 



I II III IV 



Q 25.44 23.52 24.78 19.80 



Or 21.68 25.02 22.24 7.78 



Ab 28.82 20.44 20.44 37.20 



An 15.57 15.29 '13.34 10.29 



Di .68 2.54 8.57 



Hy 4.26 9.97 6.63 3.42 



II 1.98 3.51 5.47 8.21 



Mt 1.16 1.16 2.09 2.78 



Ap 91 .91 1.86 2.17 



Pr .24 .36 



Incl 47 .64 .54 .43 



Summary of Classification 



Number. Symbol. Magmatic name. 



I T. 4. 3(2). 3. amiatose 



II II. 4. 3. 3. harzose 



III II. 4. 3(2). 3. harzose 



IV II. 4. 2. 4. dacose 

 V II. 4. 3. 3. harzose 



V 

 14.76 



22.80 

 23.58 

 18.35 



13 



.80 



2 



.28 





.93 



1 



. :>:> 



Mineralogically the rock is characterized by (1) calcic andesine as the 

 chief feldspar with notable amounts of alkali feldspar developed in part 

 as microperthite ; (2) a variable but frequently a considerable amount of 

 quartz, and (3) orthorhombic (hypersthene) and monoclinic (augite) 

 pyroxenes with very subordinate biotite, which becomes the chief mafic 

 mineral in several localities, and sometimes a little hornblende. 



From the microscopic study of a large number of thin sections covering 

 all known exposures the rock is quite uniform in mineral composition, but 

 considerable variation in the proportions of the principal minerals is 

 indicated. Confirmation of this variation is shown in the three subrang 

 positions of the rock in the Quantitative System as determined by five 

 analyses made on specimens from different localities. The norms corre- 

 sponding to the five analyses made of the rock indicate in each case more 



