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



eral instances are low down on the south-facing slopes of the ridges 

 next to the creek beds, but are not visible on the north-facing 

 slopes. This seems to be more than a mere coincidence. The dif- 

 ference of exposure on the two sides of the canyon may be due 

 partly to the thick vegetation and partly to the depth of decompo- 

 sition and the admixture of organic matter in the formation of the 

 soil on the north-facing slopes. 



In general the Miocene shales near the diabase dip away from 

 the intrusion as if it were the axis of an anticline. (See Fig. 2.) 

 This may be due to a lifting action of the diabase upon nearly hori- 

 zontal strata, or possibly to the fact that a pre-existing axis pre- 

 sented the line of weakness along which the intrusion was made. 

 There are instances of sill-like intrusions or sheets between the 

 sedimentary beds of this area. The evidence of oil well records is 

 available in some instances to show the presence of such sheets. 

 At well No. I, on San Gregorio Creek near the mouth of Harring- 

 ton Creek, the San Mateo Oil Company put down a test hole, and 

 a sheet of diabase was encountered at a depth of one hundred feet. 

 A hundred feet deeper the drill passed through the diabase and 

 again entered the shale. This well is about a quarter of a mile 

 from the igneous outcrop on Harrington Creek. Mr. Bell, on 

 whose property another well was bored about ten years ago, is 

 authority for the statement that no diabase was encountered in 

 sinking that well, which is about four hundred yards west of San 

 Mateo Oil Company's well No. i, and away from the diabase. 



Fig. 9. Northeast-southwest section through the Bella Vista oil well, San 

 Mateo County. Photograph by Ralph Arnold. 



The well sunk by the Bella Vista Oil Company on the Bella Vista 

 ranch, north of El Corte Madera Creek, encountered, according to 

 the report of the driller, a fifty-foot stratum of diabase at a depth 

 of four hundred and fifty feet ; the drill then passed into shale for 

 another hundred feet, after which it again passed through diabase 



