PETROGRAPHY 641 



Composition of the pyroxene. — Most analyses of pyroxene from the Tri- 

 assic diabase show a moderately aluminous diopside. As compared with 

 augite, there is a great excess of ferrous oxide and magnesia over lime, 

 alumina, and ferric oxide. Analyses 1 to 4 in the following table may be 

 regarded as approximately equivalent to a combination of diopside with 

 common aluminous augite in proportions ranging from about 3 : 1 to 1 : 1, 

 with small amounts of alkalies, indicating the presence of the acmite mole- 

 cule. Number 5 approaches more nearly a typical augite. 



Analyses of Pyroxene from Triassic Diabase 4 



12 3 4 5 



Si0 2 47.72 48.54 50.71 48.83 49.33 



A1 2 3 3.44 5.50 3.55 4.41 9.15 



Fe 2 3 5.93 2.77 0.27 



FeO 18.34 21.25 15.30 9.00* 9.05 



MgO 12.89 7.67 13.63 17.11 14.58 



CaO 11.40 10.97 13.35 20.51 16.36 



Na 2 0.S6 | j .... 0.55 



K 2 0.37 [ 3 - 10 1A8 \ .... 0.19 



MnO 0.81 



Ign 0.00 0.82 1.17 0.25 



100.95 100.62 100.00 99.86 99.73 



1. From coarse-grained diabase from near the middle of the sheet, Rocky 

 Hill, New Jersey. Pyroxene constitutes 41 per cent of the rock. 



2. From quartz diabase 420 feet from the upper surface of the sheet, Rocky 

 Hill, New Jersey. Pyroxene constitutes 45.6 per cent of the rock. 



3. From diabase sheet, West Rock, New Haven, Connecticut. 



4. From olivine diabase, near Chatham, Virginia. 



5. From hypersthene diabase, Culpeper County, Virginia. 



Composition of the feldspars. — The composition of the feldspars sIioavs 

 a wide range, corresponding in general to the differentiations as shown 

 by the mineral constitution of the rock. Thus in the following table 

 analyses la to Id, from quartz diabase, range from nearly pure albite to 

 andesine, associated with decreasing proportions of orthoclase; 2 and 3 

 show andesine-labradorite to labradorite from normal diabase, and 4 a 

 more basic labradorite from hypersthene diabase. From the high per- 

 centages of silica, it is evident that free quartz was present in some of the 

 specimens. Thus there is an excess of 20.6 per cent in la, 26 per cent in 

 lb, 11 per cent in 3b, and smaller amounts in the others. 



4 Numbers 1 and 2, by A. H. Phillips (Amer. Jour. Sci., vol. viii, 1899, p. 267) ; 3, by 

 G. W. Hawes (Amer. Jour. Sci., vol. ix, 1875, p. 185) ; 4, by T. L. Watson (Amer. Geol., 

 vol. xxii. 189S, p. 98) ; 5, by W. G. Brown (Bull. Geol. Soc. Am., vol. 2, 1891, p. 344 J 



* Determined as Fe 2 3 10.01. 



5 By difference. 



