1904.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 



35 



of the dikes. The surfaces of the dikes are quite irregular, giving 

 a more or less wavy line in section, but the planes of contact of 

 most of the dikes are approximately perpendicular to the bedding 

 planes of the tuff and interbedded limy layers. Some of the dikes 

 extend into the diabase which has intruded the tuff beds. Chalce- 

 dony, quartz and calcite form veins and fill cavities all through the 

 tuff, limy tuff beds and the limestone dikes. The minerals depos- 

 ited from solution are of later origin than the limestone dikes. Fig. 

 1 6 shows in detail a small section of the tuff exposed on the side 

 hill north of the Langley house. The chalcedony was deposited 

 along a fault line developed after the intrusion of the limestone into 

 the tuff. Fig. 1 7 is a photograph taken on the Searsville-La Honda 

 road a quarter of a mile north of La Honda, and shows the tuff cut 

 by limestone dikes and calcite veins. 



Fig 17. Vertical section along the Searsville-La Honda road one fourth mile 

 north of La Honda, showing limestone dikes (D) and secondary calcite 

 veins (V) in the diabase tuff. Photograph by Ralph Arnold. 



The origin of the limestone dikes is easily accounted for when 

 the relations of the containing and associated terranes is consid- 



