QUARTZ-BEARING HYPERSTHENE-ANDESINE SYENITE 217 



dant than augite in the more quartzose facies, and biotite rare. The Blue 

 Ridge type is more closely allied with rocks of dioritic than of syenitic 

 composition because of its chief feldspar being a more calcic plagioclase 

 of andesine composition, although, orthoclase is a prominent constituent 

 and in some thin sections may equal or exceed plagioclase in amount. 

 Broadly speaking, the rock is more a transitional than a well defined 

 type. For convenience of comparison the composition and ratios of the 

 normative feldspathic content of the Adirondack and Blue Eidge types 

 are given in tabular form on the preceding page. 



Summarizing the results obtained from a comparison of the normative 

 feldspathic constituent of the syenite from the Adirondacks and the Blue 

 Ridge, the Adirondack type shows a higher average of total feldspar 

 (71.29 per cent as against 60.57 per cent for the Blue Ridge), a higher 

 average total of plagioclase (44.85 per cent as against 40.66 per cent for 

 the Blue Riclge), a higher average total of orthoclase (26.44 per cent as 

 against 19.90 per cent for the Blue Ridge), and a more nearly equal ratio 

 between total orthoclase and total plagioclase (1 : 1.6 as against 1 : 2 for 

 the Blue Ridge). Not only does normative orthoclase average higher in 

 the Adirondack type, but normative plagioclase is more sodic than in the 

 Blue Ridge type. This is confirmed both by the. examination of thin 

 sections of the rocks under the microscope and by chemical analysis. 

 The above statements are based on averages of the total analyses. Ex- 

 amination of the table will indicate wide variation in each case, and indi- 

 vidual analyses may be singled out which show reasonably close agreement 

 for the rocks from the two States. 



Quartz is subject to a wide but about equal variation in both the 

 Adirondack and the Blue Ridge type, and unquestionably specimens 

 from the Adirondacks can be duplicated in the Blue Ridge, so far as the 

 amount of quartz is concerned. 



In the Adirondack type the chief mafic constituent is augite, with 

 hornblende very common, and at times a little hypersthene, but rarely 

 biotite. The Blue Ridge type is a hypersthenic rock with constant augite, 

 which in places may even equal or exceed in amount hypersthene. Horn- 

 blende is less common and biotite rare, except in one or two places, where 

 it becomes an important constituent. The ratio of mafic to felsic min- 

 erals is naturally variable, but probably no more so for the Adirondack 

 type than for the Blue Riclge type. Wide variation in both normative 

 hypersthene and diopside characterizes the rocks from both regions, but 

 the average for each is quite close for the same type in the two States, 

 although the norms of the Virginia type apparently indicate a slightly 

 higher average in each. 



