230 



JOSEPH P. IDDINGS 



with a slight difference in the oxides of iron, and a notable amount 

 of titanium oxide. It closely resembles the analysis (&) of lepido- 

 melane from nephelite- syenite from the neighborhood of Lange- 

 sundfjord, Norway, by Scheerer.' If this analysis of mica is used 

 in the calculation of the chemical composition of the rock, the result 

 is that given under IV. 



The mica is clearly a lepidomelane rich in iron and alumina and 

 poor in magnesia. When a greater variety of micas has been separated 

 from igneous rocks and carefully described and analyzed, it will 

 be possible to estimate the chemical composition of a rock from a 

 microscopical investigation with greater accuracy. 



The rock from near Llano, Tex., may be called a quartz-feld- 

 spar-porphyry having the composition of a granite. In the "Quan- 

 titative System of Classification" it is a graniphyro-liparose-alaskose. 



The norm calculated from Dr. Washington's analysis, III, with 

 the addition of fluorine determined microscopically from fluorite 

 is given below under (i). The norm calculated from the mode 

 by means of estimated Analysis IV is given under (2) : 



The rock is a persalane with about 5 per cent, of femic com- 

 ponents. Following the norm from Dr. Washington's analysis, the 

 quartz is so abundant that it is quarfelic, columbare, near the quar- 

 dofelic order britannare. It is therefore a britannare-columbare. 

 It is peralkalic of the most extreme kind, having no anorthite 

 feldspar, the lime being entirely femic, in fluorite and apatite. It 

 is alaskase near liparase, a liparase-alaskase. And with respect to 



'See W. C. Brogger, Zettschr. Kryst. Mm., Vol. XVI (1890), p. 191. 



