612 WILLIAM J. MILLER 



small scales, and rapid variations in degree of foliation, are essen- 

 tially the result of varying magmatic currents under differential 

 pressure, principally during a late stage of magma consolidation; 

 the almost universal, but varied, granulation of these rocks was 

 produced mostly by movements in the partially solidified magma, 

 and possibly in part by moderate pressure applied after complete 

 consolidation ; and the mineral flattening or elongation was caused 

 by crystallization under differential pressure in the cooling magma. 



FOLIATION OF THE ANORTHOSITE 



It is not the present purpose to discuss thoroughly the origin 

 of the structural and textural features of the Adirondack anortho- 

 site. Only a few of the more important phenomena will be briefly 

 considered. In general, the explanations above given regarding the 

 foliation and granulation of the syenite-granite series apply also 

 to the anorthosite. 



Character of the anorthosite. — With the exception of a few small 

 outlying masses, the anorthosite occupies a practically unbroken 

 area of 1,200 square miles in the central-eastern Adirondack region. 

 It quite certainly represents a single great intrusive body which 

 is older than the syenite-granite series. In its typical develop- 

 ment the rock consists almost wholly of basic bluish-gray plagioclase 

 and is very coarse-grained, the feldspars often measuring from one 

 to several inches in length. There are several important differ- 

 entiation variations of the great mass. One of these is coarse- 

 grained, but carries a considerable percentage of dark minerals; 

 another is finer-grained and more gabbroic looking, owing to dark 

 minerals, chiefly augite and ilmenite; while still another facies 

 consists almost wholly of white basic plagioclase, or such white 

 feldspar and more or less dark minerals. The great bulk of the rock 

 is highly feldspathic and practically devoid of foliated structure, 

 probably partly because of lack of minerals favorable for its 

 production or accentuation, while the more gabbroic (especially 

 finer-grained) types are almost invariably well foliated, frequently 

 excessively so. 



All of the varieties show more or less granulation, sometimes 

 to a high degree, this being particularly true of the less coarse- 



