CONCLUSIONS 799 



gions; in the typical Deccan and Oregonian basalts it is but sparingly 

 present in a few specimens. Neither hornblende nor biotite seems to be 

 present as a normal constituent in any of the regions. 



Quartz is not usually present in these basalts, although many of them 

 show an excess of silica in the norm. It occurs in many of the Palisadan 

 diabases, however, in micropegmatitic patches interstitial between the 

 pyroxene and the feldspar. Magnetite is common in all the noncrystal- 

 line plateau basalts and in decidedly large amount. Its quantity dimin- 

 ishes, however, with increase in the content of glass. This is the rule in 

 my specimens of the Deccan and Oregonian basalts and is also true of 

 the Thulean and Palisadan basalts, to judge from the descriptions of 

 Holmes and Lewis respectively. The magnetite is evidently highly 

 titaniferous, and ilmenite is present in many of the basalts, forming thin 

 plates. Apatite is common in the usual small prismoids, and calls for 

 no comment. 



In crystallinity the plateau basalts vary from entirely holocrystalline 

 to quite vitreous forms. The amount of glass may vary from none to 

 about 25 per cent, rarely more, and small, local occurrences of almost 

 holohyaline basalts may occur. The glass is either colorless or so crowded 

 with "dust" that it often appears to be opaque, or is yellow. The content 

 in augite, and still more in magnetite, decreases with increase in the 

 amount of glass, pointing to the conclusion that these two minerals are 

 the last to crystallize, and that the glass has the general composition of a 

 mixture of augite and magnetite. 



The microtexture is most often ophitic, the thin tables of labradorite 

 lying in an interstitial mass of anhedral augite grains. Less often it is 

 intersertal, and still less often is a typical "basaltic" texture shown. 

 Textural evidence of flow is seldom seen. 



Chemically, the plateau basalts differ materially from other basalts in 

 one or two features. In Table IX are given the averages of analyses of 

 basalts of various regions, with the average basalt as computed by Daly 

 from 161 analyses of basalts so named by the authors. 43 



The averages of the three most typical plateau basalt regions — the 

 Deccan, Oregonian, and Thulean — are closely alike. The percentages of 

 silica, alumina, lime, alkalies, phosphorus, and manganese oxides are 

 quite like those in the volcanic cone basalts, although both soda and 

 potash are slightly lower. Magnesia is distinctly lower, except in the 

 Thulean region and in the one analysis of a Patagonian basalt. The 

 chief difference is seen in the much higher amount of iron oxides, with 

 ferrous oxide greatly preponderating over ferric oxide. In the typical 



43 R. A. Daly : Proc. Amer. Acad., vol. 45, 1910, p. 224. 



