50 HAEHL AND ARNOLD— THE MIOCENE DIABASE. [Feb. 5, 



spaces between crystals of feldspar. 5. In large irregular patches 

 sometimes two and a half millimetres in diameter, filled with a con- 

 fusion of microlites of some indeterminable mineral. 



Chemical Characters and Analyses. 



The writers are indebted to the United States Geological Survey 

 for the analyses (I and II) of the two typical facies. For the pur- 

 pose of comparison and discussion there have been adcied analyses 

 of the analcite from Cuyamas ^ (III), the teschenite diabase (IV), 

 plagioclase feldspar (VI) and analcite (VII) from Point Sal,^ and 

 a typical analysis of labradorite from Dana (V).' 



I. II. III. IV. V. VI. VII. 



SiOj 50.12 49.60 50.55 49.61 56.0 52.72 54.40 



AljOj 18-52 16.56 20.4S 19.18 27.5 30.46 23.04 



FcjOg 2.47 4.28 2.66 2.12 0.7 



FeO 4.1 1 4.44 4.02 5.01 



MgO 2.68 5.38 4.24 4.94 0.1 



CaO 8.99 9.22 7.30 10.05 10. 1 ii.oi 0.21 



NajO 5.22 3.31 8.37 5.62 5,0 3.70 13.33 



KjO 1.46 1.25 2.27 1,04 0.4 0.42 0.19 



H2O 1.64 1.44 



H2O 3.09 2.58 0.44 Ig 3-55 144 8.46 



TiO^ 1.33 1.86 



P2O5 o. 18 0.30 0.27 



S03 0.08 0.17 tr. 



CrjO, tr. 0.03 



NiO none none 



MnO tr. 0.08 



BaO 0.02 0.06 



99.91 100.55 100.33 101.39 99.8 99.75 99.63 

 Sp. Gr 2.732 2.825 2.782 



(I) Diabase (typical diabasic facies), from one mile north of 

 Bella Vista ranch houses, San Mateo County, California. Speci- 

 men No. 24. E. T. Allen, Analyst. (U. S. Geological Survey.) 



(II) Diabase (typical basaltic facies), from Mindego Hill, San 



1 "Analcite Diabase," by H. W. Fairbanks, Bull. Dept. Geol. Univ. Cal., 

 Vol. I, p. 293, Berkeley, 1895. 



2 "The Geology of Point Sal," by H. W. Fairbanks, Bull. Dept. Geol. Univ. 

 Cal., Vol. II, pp. 1-92, Berkeley, 1896. 



3 System of Mineralogy, by J. D. Dana. Sixth edition. 



