GEOLOGY OF MOUNT MARCY 29 



some basaltic dikes and also some curious included masses as will 

 be later described. 



While the anorthosites are always predominantly plagioclase, they 

 do contain varying amounts of dark silicates, which may become 

 decidedly prominent features, especially on the borders of intrusive 

 masses. They mark passages to the normal gabbros. In order 

 to test the range in varieties of plagioclase by means of specific 

 gravities, a set of eleven specimens weighing from lo to 40 grams 

 was selected and the specific gravities were determined in distilled 

 water with a chemical balance amid the usual physical conditions 

 of a laboratory. It is impossible to get representatives entirely free 

 from inclusions or slight admixtures of heavier minerals. The 

 specific gravities which exceed those of anorthite are due to this 

 admixture.^ 



Newcomb 2.692 North Hudson 2. 711 



Essex 2.695 Schroon 2.714 



Aiden Lair 2.696 Locality lost 2.717 



Keene valley 2.696 Elizabethtown 2.736 



Lake Placid 2.708 Locality lost 2.770 



Locality lost 2.803 



It is evident that the greater number are a little below or a little 

 above 2.700. The plagioclases range as follows : 



Albite Abi Ano 2.605 Labradorite Abi An2 2.710 



Oligoclase Abs Ani 2.649 Bytownite Abi An^ 2.733 



Andesine Ab2 Am " 2.660 Anorthite Abo Ani 2.765 



Andesine-Labradorite transition Abi Ani 2.679 



Determinations of this sort are believed to be more comprehensive 

 than observations of extinction angles in scattered thin sections. 

 The anorthosite may therefore be considered predominantly or essen- 

 tially labradorite. The component crystals are sometimes coarsely 



1 In the valuable paper " Notes on the Litholo^ of the Adirondacks," 

 13th Annual Report, N. Y. State Museum, p. 86, 1876, the late Prof. Albert 

 B. Leeds gives the specific gravities of forty-two specimens called norite, 

 under which name the labradorite and other plagioclase rocks, chiefly from 

 the Keene valley, are included. The values range from 2.67 through 3.24. 

 An additional one of diallage at 3.386, and one of hypersthene at 3.459 are 

 given. Since anorthite, the most basic and heaviest of the feldspars is 2.765, 

 all values above this (no. 11 in Doctor Leeds's series) must have other heavier 

 minerals. Analysis no. i of anorthosite as given two or three pages h^er 

 from Doctor Leeds is no. 8 in his list, and analysis no. 2 is his no. 4._ The 

 percentages of other iand heavier minerals than feldspars, as given in the 

 results of recasting, will throw some light on the extent to Avhich higher 

 specific gravities call for p^-rcxenes, garnet, ilmenite and other heavier 

 minerals. Evidently basic p-abbros or basic syenites as we now know the 

 rocks, must be included in Doctor Leeds's list. Unfortunately he gives the 

 localities of only the two or three analyzed, so that we can not trace the 

 others. 



