SEGREGATION GRANITES 163 



cavities were "filled with the residuum of the molten magma. 

 Enough of the rock was formed to make it perfectly solid, for no 

 further motion could take place without disturbing the micropeg- 

 matite borders of the felspar laths and fracturing the excessively 

 delicate apatite needles. The remaining interstices seem, agreeing 

 with the general law of increasing acidity in residual magmas, to 

 have been filled with the final concentration of an acid aqueo- 

 igneous magma which had been corroding the olivine and forming 

 the less basic augite from it. In this magma were also concentrated 

 the absorbed gases, aqueous and otherwise, which the dike margin 

 originally contained and which, as the dike solidified at the margin, 

 would probably be driven from it and concentrated at the center. 



"The acid magma thus left seems to have proceeded to produce 

 brown hornblende upon and out of the augite; brown mica upon 

 and out of the iron oxides, as Smyth has suggested; and pegmatite 

 growths on or out of the feldspar, while apatite needles formed 

 across the cavities." The convincing arguments were that these 

 interstices did not occur in marginal sections, and showed no signs 

 of being more abundant in uralitic sections, but were best developed 

 in otherwise fresh dikes, and were not present with superficial 

 textures such as the amygdaloid textures. 



The facts I have seen in the Medford diabase and elsewhere 

 in the intervening thirty years have only strengthened the convic- 

 tion that this interpretation is substantially correct, though it 

 should be expressed not so much in terms of acid and basic, as in 

 terms of those eutectic lines and troughs which the geophysical 

 laboratory at Washington has worked out, the theoretical bearing 

 of which has been developed by N. L. Bowen.' 



The fluid magma from which the minerals of these interstices 

 crystalKzed out would be closely held by capillarity if they were 

 not large. But with increasing coarseness and size of interstices 

 there would be more chance for it to drain out like honey from the 

 honeycomb, or as Bowen and Harker have suggested, be squeezed 



' "The Problem of the Anorthosites," Jour. GeoL, Vol. XXVIII (1919), pp.393- 

 434; ibid., Vol. XXV, 3 (1917), pp. 210-44, "Crystallization — Differentiation in Igne- 

 ous Magmas," and literature there cited; Am. Jour. Sci. (1915), p. 4o7> (1914), 

 p. 207, etc. 



