K04.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 17 



Field Relations. 



General Relations of the Diabase. , 



The exposures of the diabase have been traced for a distance of 

 approximately thirty-two miles in a generally southeasterly direc- 

 tion from a point near the San Mateo-Halfmoon Bay road, on Pil- 

 arcitos Creek, in San Mateo County, to a point on the head- 

 waters of Lompico Creek, four miles east of the town of Boulder 

 Creek, in Santa Cruz County. There is also a basaltic out- 

 flow exposed near Stanford University, which is probably closely 

 related genetically to the diabase. The exposures east and south of 

 those found at the head of Devil's Canyon are not shown on the 

 map, as, with one exception, they are of minor importance. The 

 largest exposed areas of the diabase are in the vicinity of Langley 

 and Mindego Hills east of La Honda, and on the ridges between 

 the headwaters of Pescadero Creek and the San Lorenzo River (the 

 latter being outside of the area shown on the accompanying map). 



PROC. AMER. PHILOS. SOC. XLIII. 175. B. PRINTED MARCH 9, 1904. 



