THE THERALITB-BASANITE GEOUP 



89 



sanidin, hornblende, biotite, hauyne, melanite, perowskite, and 

 a mineral of the spinel group. 



Chemical Composition. — The following is the composition of 



(I) a nepheline tephrite from Antao, Pico da Cruz, Azores, and 



(II) a nepheline basanite from San Antonio, Cape Verde 

 Islands, as given by Eoth.^ 



Chemical Composition of Tephrite and Basanite 



Constituents 



Silica (SiOs) . , . . 

 Alumina (AI2OS) . . . 

 Iron sesquioxide (FegOs) 

 Iron protoxide (FeO) . 

 Magnesia (MgO) . . . 

 Lime (CaO) .... 

 Soda (NasO) .... 

 Potash (K2O) .... 

 Water (H2O) .... 



II 



43.09% 



17.45 



18.99 



• • • « 



4.63 



9.76 



6.02 



1.81 



0.33 



101.08% 



Structure. — The rocks of this group are as a rule porphyritic 

 with a holoerystalline ground-mass, though sometimes there is 

 present a small amount of amorphous interstitial matter or 

 base; at times amygdaloidal. 



Colors. — The colors are dark, some shade of gray or brownish. 



Classification and Nomenclature. — According to their vary- 

 ing mineral composition Eosenbuseh divides them into : Leucite 

 tephrite, Leucite basanite, Nepheline tephrite, Nepheline ba- 

 sanite. 



The group, it will be observed, stands intermediate between 

 the true basalts and the nephelinites to be noted later. Their 

 distribution, so far as now known, is quite limited. 



7. THE PEEIDOTITE-LIMBXTEaiTE GROUP 



This and the following groups include eruptive rocks in 

 which neither quartz nor feldspars of any kind longer appear 

 as essential constituents. They are therefore very low in silica, 

 and classed as ultrabasic. Although in most cases comparatively 

 insignificant as rock masses, they are peculiarly interesting as 



*Abliandlungen der Konig. Akad. der Wissenseliaften zu Berlin, 1884, 

 p. 64. 



