1904.J 



HAEHL AND ARNOLD — THE MIOCENE DIABASE. 



27 



An interesting section (shown in Fig. 8) is exposed beside an old 

 road one-half mile north of the Langley ranch house. The diabase 

 at that place breaks through the Miocene shales, following the bed- 

 ding planes in a general way, but sometimes breaking through the 

 beds. Small inclusions of the shale are found in the diabase, but 

 no alteration of either the inclusions or beds thus intruded was 

 noticed. 





Fig. 8. Vertical section of bank on south side of road one-half mile north of 

 Langley's ranch house, showing diabase intrusion in shale. The shale, 

 which is unaltered, dips into the bank at an angle of 35°. 



The Purisima beds (Pliocene) which cover large areas of the dia- 

 base are not penetrated by the diabase. This may explain the pres- 

 ence of only small, isolated patches of the diabase along the north- 

 ern end of the area. Either the Miocene or post-Miocene denu- 

 dation over the northern end of the diabase area must have been 

 great, or else the Monterey shale must be represented by sandstone 

 over that territory, for there, wherever the diabase is exposed, it is 

 in the Vaquero sandstones, while the Monterey shale appears to be 

 almost lacking. Over this tract a deposition of the Purisima sedi- 

 ments took place after the denudation and covered large areas of 

 the eroded surface of the diabase. At the base of the Purisima 

 beds are conglomerates made up largely of diabase pebbles, and 

 these conglomerates are now exposed in the canyons together with 

 small areas of the diabase in place. The presence of the diabase 

 conglomerate at the base of the Purisima formation, together with 

 the fact that the diabase is intrusive in the Miocene, establishes the 

 time of at least the greater part of the igneous intrusions as later 

 than the middle Miocene (Monterey), and before the Pliocene 

 (Purisima). It is noticeable that the exposures of diabase in sev- 



