36 NEW YORK STATE MUSEUM 



anorthosite to have lifted rather than to have extensively cross-cut 

 or engulfed Grenville strata; and failure of both syenite-granite 

 and later gabbro to penetrate the southwestern half of the great 

 anorthosite body, and moderate penetration of the northeastern 

 half by the rocks just named, thus very strongly suggesting a lacco- 

 lith very thick toward the* southwest and relatively thin toward 

 the northeast. 



Probable origin of the anorthosite by settling of femic minerals. 

 The writer's conception is that the anorthosite resulted from the 

 settling of femic constituents in an originally gabbroid intruded or 

 intruding magma. This is fundamentally the view expressed by 

 Daly 1 who says : " The anorthosites of the world are best regarded 

 as ... gravitative differentiates of gabbroid magma " usually in 

 laccoliths. Regarding differentiation in general by sinking of crys- 

 tals, F. W. Clarke 2 says: "Gravitative adjustment is presumably 

 most effective in slowly cooling magmas, especially when partial 

 crystallization has occurred. The minerals first formed must have 

 time to sink. The rate of cooling, therefore, is a distinct factor in 

 the differentiation of igneous rocks/' There is every reason to think 

 that the great igneous body of Adirondack anorthosite cooled very 

 slowly. 



Very briefly stated, the writer considers the main steps in the 

 development of the anorthosite to have been as follows : first, intru- 

 sion of a laccolithic body of gabbroid magma only somewhat 

 greater across than the exposed area of the anorthosite; second, 

 relatively rapid cooling of the marginal portion to give rise to the 

 chilled gabbroid border phase; and, third , settling of many of the 

 slowly crystallizing femic minerals in the still molten interior por- 

 tion of the laccolith, leaving a great body of magma to crystallize 

 gradually into anorthosite. Thus at the bottom, and probably 

 nowhere visible in the field, lies a mass of pyroxenite or peridotite ; 

 next above it the thick body of Marcy anorthosite ; and at the top 

 and on the outer margins the chilled gabbroid border fades known 

 as Whiteface anorthosite. 



The border facies thus merely represents the very outer and 

 upper portions of the original gabbroid magma which solidified too 

 rapidly to permit much settling or separation of femic minerals 

 from it. This marginal phase came into direct contact with the 



1 Igneous Rocks and Their Origin, p. 229-43. 1914. 

 *U. S. Geol. Survey Bui. 491, p. 297. 1911. 



