404 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



of that in analcite basalt from Colorado,* the analysis of which gives essential 

 oxides in the following percentages: Si0 2 , 49-26; A1 2 3 , 6-01; Fe 2 3 , 3-31; 

 FeO, 4-23; MgO, 12-40; CaO, 21-79. The iron oxide not entering into the 

 constitution of the augite is regarded as contained in the titaniferous magnetite. 

 The calcium of calcite is considered as derived from the feldspar; the corres- 

 ponding C0 2 is subtracted from the total. 



Recalculating the analysis after these eliminations we have: — 



Per cent. Molecules. 



Si0 2 57-98 -966 



A1 2 3 25-28 -248 



CaO 2-74 -049 



SrO -93 -009 



BaO 1-28 -008 



Na,0 5-24 -084 



K 2 6-55 -070 



100-00 

 The analysis shows that the feldspar is relatively rich in barium and stron- 

 tium oxides and seems to confirm the view that the peculiar optical properties 

 are to be correlated with the presence of those oxides. 



The molecular proportions in the ' purified ' feldspar correspond to the 

 following proportions among the four recognized feldspar molecules and a hypo- 

 thetical ' strontium feldspar ' molecule which is analogous to that of celsian. 



Albite 42 



Orthoclase 36 



Anorthite 12 



Celsian 5 



"Strontium feldspar" 5 



100 

 It should be noted that, since a certain though small proportion of the outer 

 shells (composed of apparently pure orthoclase) of some of the phenocryst frag- 

 ments are included in the analyzed sample, the proportion of the potash mole- 

 cule is somewhat higher than it would be found in the anomalous cores of the 

 phenocrysts. The core feldspar (making up, as estimated, about 80 per cent of 

 the average phenocryst) is clearly an unusual species, carrying, as it does, essen- 

 tial amounts of five different bases. It is perhaps best described as an abnormal 

 anorthoclase, rich in the barium-strontium feldspar molecules. 



Other Constituents. — The phenocrystic pyroxene is greenish-black in the 

 hand-specimen and very pale green in thin section. It is not sensibly pleo- 

 chroic and has the optical characters of common augite. It incloses magnetite 

 and apatite but is itself of earlier generation than the phenocrystic feldspar. 

 The biotite is apparently a common, highly ferruginous variety of mica, with 

 intense pleochroism in colour-tints from pale to very deep leaf-brown. The 

 rather rare olivine phenocrysts are everywhere entirely altered to brownish- 

 yellow serpentine and dolomite but the shape and general habit of the 

 pseudomorphs leave little doubt as to the nature of the original mineral. 



♦Bull. 228, U.S. Geol. Survey, p. 165 and Jour. Geology, Vol. 5, 1897, p. 684, 



