408 AY. J. MILLER ADIEOXDACK AXORTIIOSITE 



anorthosite; (2) meager evidence of syenite intrusions in anorthosite, 

 and (3) common occurrence of intimately mixed Grenville and syenite, 

 but absence of intimately associated Grenville and anorthosite. 



SiGNIFICAXCE OF THE VARLIBLE COMPOSITIOX AXD STRUCTURE OF THE 



AXORTHOSITE 



According to Bowen, "anortliosites are made up almost exclusively of 

 the single mineral plagioclase," and therefore ^^the conception of the mu- 

 tual solution of minerals in the magma and the lowering of temperatures 

 consequent thereon is no longer applicable." 



My experience in the field, with this idea of the composition of the 

 Adirondack anorthosite definitely in mind, has convinced me that the 

 rock is by no means an almost perfectly homogeneous mass of plagioclase. 

 The main bulk of the Marcy anorthosite contains 5 to 10 per cent of 

 minerals other than plagioclase. Portions with about 10 per cent are 

 common, and in many places there are 10 to 20 per cent, or even more, 

 of dark minerals. It is also true that some portions of the mass contain 

 less than 5 per cent of dark minerals visible to the naked eye. Conserva- 

 tively estimated, I believe the average Marcy anorthosite carries fully 10 

 per cent of minerals other than plagioclase. 



In my field-notes on both the Lake Placid and Schroon Lake quad- 

 rangles, I have recorded many observations of anorthosite-gabbro and 

 more typical anorthosite exhibiting perfect gradations from one into the 

 other. Such gabbroid facies exist locally throughout the anorthosite 

 body, in many places as rather distinct zones or belts a few feet or rods 

 wide, and in other places on much larger scales. Many other gabbroid 

 portions are much more irregular in shape and not so distinctly separated 

 from the purer anorthosite. 



Similar irregularities in the Morin anorthosite near Montreal have 

 been described by Adams,^^ who says : 



"This irregularity is sometimes scarcely noticeable, but is at other times 

 striking, and is due to concentration of the bisilicates and iron ore in some 

 parts of the rock. The portions richer in bisilicates may take the form of 

 large irregular shaped patches occurring at intervals through the rock, or of 

 many small patches occurring abundantly in certain parts of the rock which 

 elsewhere is nearly free from them. In some cases these are arranged to form 

 irregular wavy streaks instead of patches. Sometimes these patches are rudely 

 parallel, giving a sort of strike to the rock, but in other places they are quite 

 irregular in arrangement. Between these patches or streaks rich in bisilicates, 

 and rather badly defined against them, are portions of the rock which are very 

 poor in or often quite free from bisilicates." 



13 F. D. Adams : Geol. Sur. Can., Guide Book No. 3. 1913. p. 10. 



