ELIZABETHTOWN AND PORT HENRY QUADRANGLES 29 



The variety of the plagioclase is best shown by the chemical 

 analysis later cited but it can also be determined in an approximate 

 way by means of the extinction angles. Measurements of specific 

 gravity would also be significant but they have not been used as 

 the above tests were esteemed sufficient. The plagioclase lies most 

 often just beyond the labradorite ranges of Ab^ An^ to Ab^ An2'^, 

 yet short of the bytownite of Ab^ Aug. 



In the more basic varieties we find the upper limits of the by- 

 townite series also represented. 



Under the microscope and when the original texture of the rock 

 has not been crushed and destroyed, the thin sections show between 

 crossed nicols the characteristic twinning of the plagioclases in a 

 remarkable degree of perfection. For purposes of instruction few 

 rocks are so well adapted for illustrating these phenomena. At 

 times the bands cross the crystals with mathematical regularity and 

 perfection ; again they interpenetrate and pinch out like interlocked 

 fingers and hands. Even with low powers the plagioclase reveals 

 the very minute dusty inclusions, which in somewhat sparse ar- 

 rangement are distributed throughout the clear mineral. With high 

 powers the dust is seen to be in largest part an opaque to dark 

 brown mineral, in prismatic or tabular form according to the orien- 

 tation and doubtless ilmenite. Rarer pale green fragments are 

 probably diopside and spinel. The inclusions may be strung out in 

 lines parallel to the main twinning. They are never abundant 

 enough to affect the transparency of the slide in any appreciable 

 degree and in this respect they are inferior in amount to those in 

 the basic gabbros to be later described. 



1 In the algebraic designation of the plagioclases, they are considered 

 combinations of the albite molecule, Na20, AI2O3, 6Si02, written Ab, 

 and the anorthite CaO, Al^On, 2Si02, written An. The following varie- 

 ties are recognized by students of rocks. 

 Albite Abi Ano through Abs Ant 



Oligoclase Abe Aoi through Ab2 Am 



Andesine Abs An2 through Ab4 An.? 



T.abradorite Abi Ani through Abi An2 

 Bytownite Abi Ans through Abi Ane 



Anorthite Abi Ans through Abo An, 



Although the above is the assignment of species generally given in the 

 textbooks it is not a good one, since there are uncovered gaps between 

 each group. Abi Aut for example is not provided for. It should read 

 Albite Abi Ano to Abs An,; Oligoclase Abs Ani to Abo An,; Andesine 

 Ab2 An, to Ab4 An:,; Labradorite Ab4 Ann to Abi Ano; Bytownite Abi An^ 

 to Abi Ann: Anorthite Abi Ana to Abo Ani. 



