702 THE JOURNAL OF GEOLOGY. 



Point, show it to be a little more acid than the latter, though not 

 enough so as to cause it to be placed in a position in the plagio- 

 clase scale far removed from that of the feldspar of the diabase x . 

 The corresponding figures for the two plagioclases are : 



SiO s A1 2 3 Fe s 3 FeO CaO Na s O 



Gabbro 51.89 29.68 .69 12.62 3.87 



Diabase 53.75 30.39 1.26 10.84 3-76 



The augite is generally older than the plagioclase, although 



the latter mineral seems sometimes to mould the contours of the 



former one. The pyroxene occurs either in the interstices 



between the labradorite grains, or as narrow rims around the 



olivine, forming a mantle that surrounds these and separates them 



from the feldspar (see Fig. i). 2 The mineral is very light 



colored, sometimes being almost colorless, but it is usually tinged 



Fig. 1. Section of the olivine-gabbro, exhibiting the tendency of the pyroxene to 

 include olivine grains. Section 1103. X 20. 



with pink. It is moreover possessed of a diallagic parting, 

 accentuated by dark decomposition products, the most abundant 

 of which are tiny, irregular black and brown dots. These are 

 scattered everywhere throughout the pyroxene, but are accumu- 

 lated most thickly in the neighborhood of the cleavage lines. In 

 some of the pyroxene pieces are the peculiar platy inclusions 



1 Bull. U. S. Geol. Survey, No. 109. 



2 Cf. M. E. Wadsworth, Bull. No. 2, Geol. and Nat. Hist. Survey of Minn., PI. III. 

 Fig. 1. In this figure the author pictures a pyroxene and olivine bearing the same 

 relation to each other as the diallage and olivine shown in Fig. 1 of this paper. 



