74: 



BOCKS PORMEB THEOUGH IGNEOUS AGENCIES 



perowskite. Calcite, epidote, chlorite, analcite, and sundry min- 

 erals of tlie zeolite group occur as secondary products. 



Professor W. S. Bayley has computed^ the relative propor- 

 tions of the various constituents in the elaeolite syenite of Litch- 

 field, Maine, as follows: Elaeolite, 17%; potash feldspar, 27%; 

 albite, 47% ; eanerinite, 2% ; and black mica (lepidomelane), 7%. 



Chemical Composition. — The composition of the nepheline 

 syenite from several well-known 'localities is given below: 



Chemical Composition of Nepheline Syenite 



Constituents 



Algrave, 

 Portugal 



Hot Springs, 



Arkansas 



LlTCHPIELD, 



Maine 



Beembrvillk, 

 New Jersey 



Silica (SiOs) 



Alumina (AI2O3) .... 

 Ferric oxide (FezOs) . . . 

 Ferrous oxide (EeO) . . . 



Magnesia (MgO) 



Manganese oxide (MnO) . . 



Lime (CaO) 



Soda (NasO) 



Potash (K2O) 



Titanium oxide (TiOa) . . 

 Phosphoric anhydride (P2O5) 

 Water (H2O) 



64.61 % 

 22.07 



2.33 



2.60 



0.88 



2.61 

 7.68 

 6.46 

 0,09 

 0.16 

 1.13 



69.70% 

 18.85 

 4.85 



0.68 



1.34 

 6.29 

 6.97 



. . * • 



1.88 



60.39% 

 22.61 

 .42 



2.26 



0.13 



0.08 



0.32 



8.44 



4.77 



• • • « 



'.67 



50.36% 

 19.84 



}6.94 



* • * * 



0.411 

 3.43 

 7.64 

 7.17 



• • • • 

 ft » • • 



3.512 (loss) 





99,31 



99.56 



99.89 



99.303 



The essential points to be noted are the larger percentages 

 of the alkalies over those yielded by syenites of the ordinary 

 type, or the granites. 



Color. — The colors are light to dark gray, and sometimes 

 reddish. 



Structure. — The syenites, like the granites, are massive holo- 

 crystalline granular rocks, and as a rule sufficiently coarse in 

 texture to allow a determination of their essential constituents 

 by the unaided eye. In the Litchfield (Maine) syenite the 

 elaeolite often occurs in crystals upwards of 5 centimetres in 

 length, and zircons 2 centimeters in length are not rare. Neither 

 of the essential constituents occur in the form of perfect crystals, 

 while the apatite, zircon, black mica, and pyroxenes often pre- 



^ Bull. Geol. Soc. of America, Vol. Ill, 1892, p. 231. 



