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University of California. 



|VOL. 2. 



pale feldspathic veins." Their description of the banded portion is 

 as follows: "Each of these banded sheets consists of many parallel 

 layers of lighter and darker material. The component layers vary 

 in thickness from mere pasteboard-like laminae to beds a yard or 

 more in thickness. On a single exposed face of rock they may be 

 seen to be as parallel, regular, and continuous as sedimentary depos- 

 its. An intimate union exists between the material of the succes- 

 sive bands. They are welded into each other by the mutual pene- 

 tration of their component minerals." In the locality described the 

 authors have not noticed any lineation of the individual crystals, 

 although in some of the gabbros of the Inner Hebrides there is 

 present a flow structure with drawn-out feldspars. "The more 

 basic portions are not confined to the margin of the masses, but 

 alternate with the more feldspathic portions to form the banded 

 complex." They believe that the anorthosite rocks of Canada 

 originated under much the same conditions. In conclusion the 

 statement is made that "the banding is the result of the intrusion 

 as sills of a heterogeneous magma, the heterogeneity produced by 

 causes operating elsewhere and probably at lower levels in the 

 earth's crust. Then came the intrusion of the heterogeneous 

 magma as sills, and it was by the deformation of the molten mass 

 during intrusion that the banded structure was produced." 



It is the opinion of the author that the banding at Point Sal is 

 in large part due to other causes than that of a differentiated magma 

 which subsequently underwent differential movement; that this was 

 concerned, but that it alone could not have produced the phe- 

 nomena. 



The most perfect banding at Point Sal resembles closely that in 

 other regions which have been referred to, and generally considered 

 to represent the mingling of heterogeneous magmas during move- 

 ment, but the fact that here there is plainly to be discerned transi- 

 tions from the regular and even banding to that of discontinuous 

 bands, then to the gneissic structure, and finally to the massive 

 condition, leads irresistibly to the conclusion that here at least the 

 different portions of the rocks in which banding is pronounced 

 solidified from what was a fairly homogeneous magma previous to 

 movement. There was, however, a tendency to the segregation on 



