1904.J 



HAEHL AND ARNOLD — THE MIOCKXE DIABASE. 



31 



Another interesting example of the relation between different 

 facies of the tuff is shown in Fig. ii, which represents a section 

 through the top of Mindego Hill. Here the angular tuff is over- 

 lain by a water- worn tuff, which in turn grades by easy stages into 

 a siliceous sandstone. The line of demarkation between the angu- 

 lar and water-worn tuffs is very distinct, the former being dark 

 colored and grading into an almost massive basalt below, while the 

 latter is composed of well-worn fragments of light- colored weath- 

 ered amygdaloidal basaltic diabase. The water-worn layer grades 

 into a tuff, which is composed of fragments of rock replaced by 

 chalcedony, and then into a fossiliferous sandstone in which some 

 of the fossils a. id much of the rock have been replaced by chalce- 

 dony. Chalcedony and quartz veins and chalcedony-lined cavities 

 are common in the beds above the typical water-worn tuff. 



A peculiar tuff, composed of water-worn pebbles of the basaltic 

 diabase imbedded in a fine, brown, ash-like matrix, is exposed on 

 the Searsville-La Honda road just south of the mouth of Langley 

 Creek. Where weathered this tuff so much resembles a true dia- 

 base containing pebbles of basalt that at first its origin was quite 



Fig. 12. {a) Showing weathered and (/^) fractured surface of the typical imy 

 tuff from the hill north of the Langley ranch house. Reduced one-half. 

 Photograph by Ralph Arnold. 



