1904.] 



HAEHL AND ARNOLD — THE MIOCENE DIABASE. 



25 



the bedding planes of the shale in the form of a sheet. There are 

 some very striking exceptions to the sheet-like occurrences of the 

 diabase, but in general the ready cleavage of the shale along the 

 bedding planes seems to have offered the line of weakness which 

 the intrusive rock followed. Fig. 5 shows a characteristic case of 

 the diabase breaking through the Miocene shales and sandstones. 

 The shale in the middle of the dike in this exposure is slightly 

 darker colored and somewhat harder than the shale beneath the 

 diabase. The sandstone was not affected by the intrusive rock. 



Fig. 5. Vertical section exposed in small ravine od Dornberger's ranch, near the 

 Page Mill road summit, showing diabase intrusive in shale and between 

 shale and sandstone. 



Inclusions of sandstone and shale are plentiful and vary from the 

 size of a walnut to masses of hundreds of tons, but no evidence of 

 the alteration of the sandstone has been noted, except in the case 

 of the underlying beds of the basalt near Stanford University. 

 Some well-preserved vertebrate bones and teeth ( Oxyrhina fiimula 

 Agassiz) were found in a sandstone inclusion two feet in diameter 

 about one-half mile north of the Alpine schoolhouse. Fig. 6 shows 

 an inclusion of light yellow sandstone found near the edge of a 

 large diabase dike on the Searsville-La Honda road. Neither the 

 intruded sandstone, which is seen in the upper right-hand corner 

 of the picture, nor the inclusion is in the least altered. 



The inclusions of shale are usually somewhat metamorphosed, 

 but the metamorphism is not radical, changes in color and texture 

 being the chief phenomena. An inclusion of shale four inches 

 thick metamorphosed to a hard, brittle flint was found in the dia- 

 base on Oil Creek. Similar occurrences were noted at several other 

 localities in the area under discussion. Fig. 7 shows a shale layer 

 which has been slightly hardened by an intrusion of diabase. This 

 is an example of a somewhat common phenomenon. 



