1904.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 47 



The matter of the presence or absence of the nepheline was made 

 the object of particularly careful search, as its presence, if estab- 

 lished, would materially assist in accounting for the soda necessary 

 to the formation of analcite, as has been observed by Fairbanks in 

 dealing with a very similar rock in San Luis Obispo County.* As 

 shown above, however, it is very doubtful whether any nepheline 

 occurs in either facies of the diabase of the area here under discus- 

 sion. 



Pyroxene. — The pyroxenic constituent is usually augite, but en- 

 statite is occasionally noted. The augite occurs, in general, in two 

 ages — a more or less porphyritic series which are occasionally idio- 

 morphic and frequently absent entirely in the slides, and a series of 

 small allotriomorphic grains filling the interstices between the feld- 

 spars and olivines of the ground mass. Augites of this latter type 

 are seldom over five-hundredths of one millimetre in diameter, and 

 seem to be identical in composition and optical phenomena with 

 the porphyritic type. No grouping of either type could be detected, 

 the only instance of a perceptible order of arrangement being found 

 in the slides from one small area on Harrington Creek, where por- 

 phyritic augites with distinct micropoikilitic structure were ob- 

 served. The included crystals were particularly fresh plagioclases, 

 which made up about fifty per cent, of the cross section of the py- 

 roxene host. The phenocrysts seldom attained a large size in this 

 facies and were usually broken by mechanical strains or rounded and 

 etched by chemical action. However, elongated crystals with ap- 

 proximately idiomorphic outlines were not uncommon. The augite 

 is of the same pale brown to pinkish tint noted in the diabasic 

 facies. Like it, too, it is but slightly pleochroic, except for some few 

 scattered individuals whose pleochroism is somewhat marked. Po- 

 larization colors are very brilliant. Twinning according to the aug- 

 ite law is not uncommon. Only simple twins were noted. Inclu- 

 sions of glass, gas bubbles and magnetite were noted in the porpliy- 

 ritic crystals. 



Enstatite is found in a few instances in irregular plates showing 

 low interference colors. The crystals were in no case large, two- 

 tenths of one millimetre being the greatest diameter measured. 

 The surfaces of the crystals were distinctly pitted, but no distinct 



i«« Analcite Diabase," by H. W. Fairbanks, Bull, Dept. Geol. Univ. Ta/,, Vol. 

 I. PP- 273-3CO, Berkeley, 1895. 



