ORIGIN BY DIFFERENTIATION 457 



same as that of the present rock, and whose margin, in which femic con- 

 stituents tended to accumulate, developed as a chilled rather gabbroid 

 border. As already shown in this paper, I see no difficulty in assuming 

 that this anortliosite may actually have been sufficiently molten as such 

 to have been laccolithically intruded, but there is a real difficulty in try- 

 ing to account for the tendency of the femic constituents to gather in the 

 marginal portion. 



My conception is that the anorthosite resulted from the settling of 

 femic constituents in an originally gabbroid magma. This is funda- 

 mentally the view expressed by Daly,*^ who, after considering many of 

 the w^orld's best known laccoliths and sills, says: "The anorthosites of 

 the world are best regarded as . . . gravitative differentiates of gab- 

 broid magma" usually in laccoliths. Eegarding differentiation of mag- 

 mas in general by sinking oi crystals, Clarke*^ says : "Gravitative adjust- 

 ment 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 believe 

 that the great igneous body now represented by the anorthosite cooled 

 very slowly. 



Very briefly stated, I consider the main steps in the development of the 

 anorthosite to have been as follows : First, intrusion 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 por- 

 tion 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 portion of the laccolith, leaving a great body of magma to grad- 

 ually crystallize 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 facies. 



The chilled border phase merely represents the very outer and upper 

 portion of the original gabbroid magma which solidified too rapidly to 

 permit much settling of femic minerals from it. As Daly says: "The 

 contact phase, a more or less continuous shell, thus represents the original 

 magma." This marginal phase, of course, came into direct contact with 

 the country rock (Grenville) and at first, when it was in its most highly 

 fluid state, attacked the country rock with sufficient force to engulf por- 

 tions of it, send some dikes into it, and even intimately penetrate it in 



*8 R. A. Daly : Igneous rocks and their origin, 1914, pp. 229-243. 

 « F. W. Clarlie : U. S. Geol. Sur. Bull. 491, 1911, p. 297. 



