1901.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 51 



Mateo County, California. E. T. Allen, Analyst. (U. S. Geolog- 

 ical Survey.) 



(III) Analcite diabase from Cuyamas, San Luis Obispo County, 

 California. V. I^enher, Analyst. (Fairbanks. ) 



(IV) Augite-teschenite from Point Sal. Santa Barbara County, 

 California. (Fairbanks.) 



(V) Labradorite, typical analysis, Dana, System of Mineralogy, 

 Sixth Edition, p. 337. 



(VI) Feldspar from Augite-teschenite, Point Sal, Santa Barbara 

 County, California. (Fairbanks.) 



(VII) Analcite from Augite-teschenite, Point Sal, Santa Barbara 

 County, California. (Fairbanks.) 



The close relation between the two facies, as far as chemical com- 

 position is concerned, is very evident. A striking similarity exists 

 also between them and the two analyses of very similar rocks from 

 San Luis Obispo County, described by Fairbanks as analcite diabase 

 and augite-teschenite. Hand specimens and a few slides of these 

 latter rocks which were studied for comparison tend to emphasize 

 this similarity, and to make it reasonably certain that the rocks are 

 very closely related in all their properties. In that connection it 

 seems that the evidence gathered by Fairbanks,^ in dealing with the 

 probable origin of the analcite in rocks which are of this same type, 

 is particularly applicable here. Fairbanks' analyses both show a 

 slightly greater percentage of soda. Optically his feldspars agree 

 with those encountered here. Chemically they are very like the 

 typical labradorite of Dana (^ ). Nothing new beyond the data 

 given by Dr. Fairbanks in his discussions was discovered in the 

 examination of the rocks herein described. The conclusions 

 reached by that author, however, are but vaguely substantiated. 

 The presence of nepheline, at some time in the history of the dia- 

 base, has not been proven. Aside from the fact that analcite is 

 present, and that the soda necessary to permit of its formation 

 could not have come from a concentration of that element from 

 the feldspars alone, to the extent that the entire rock should show 

 a percentage of soda equal to that of labradorite, there is nothing 

 to suggest the presence of nepheline at any time. The presence 

 of the analcite chiefly within the feldspars themselves would point 



1 "Geology of Point Sal," Bull. Dept. Geol. Univ. CaL, Vol. II, p. 30; and 

 "Analcite Diabase," Bull. Dept. Geol, Univ. Cal., Vol. 1, p. 293, Berkeley, 

 1896. 



