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



The whole area presents at the surface a chain of more or less con- 

 nected patches of diabase, extending approximately parallel to the 

 coast, and also parallel to, but southwest of, the major axis of the 

 Santa Cruz Mountains. The continuity of the patches is hidden 

 by overlying strata and by dislocated masses of country rock and 

 soil to such an extent that the exact relations of the various fades 

 are difficult to ascertain. 



It is possible, however, to determine the age of the igneous mass 

 by its relation to the sediments about it. The relations of the 

 stratified rocks of the area under discussion are as follows : ^ 



r Purisima formation with Astrodapsis n. sp., Lucina acuii- 

 Pliocene -j lineata, Nucula castrensis, Pecten 3 n. sp., A, B, and C, 

 \Rostellaria undulaia and Saxidomus gibbosus. 



r Monterey shale with A?'ca montereyajia, Callisfa angus- 

 } tifrons, Pecten peckhami^ and Tellina congesta. 

 1 Vaquero sandstone with Agasoma barkerianujn, A. 

 I kernianumj Pecten magnolia^ and Turritella hoffmani. 



Associated Sedimentary Formations. 



The diabase proper breaks through beds of lower, and perhaps 

 middle, Miocene age ; while the associated diabase tuff is interbed- 

 ded with strata containing a typical lower Miocene fauna and lies 

 below the Monterey shale. The basalt'^ outflow exposed near Stan- 

 ford University overlies and metamorphoses beds of lower Miocene 

 age, and is overlain by beds containing a fauna very similar to the 

 underlying strata. This evidence indicates the lower Miocene age 

 of the basalt and its probable contemporaneousness with the dia- 

 base tuffs of Mindego and Langley Hills. Both the intrusive dia- 

 base and the tuff are in many places overlain by the Purisima 

 (lower Pliocene) beds, which show a distinct erosion line at their 

 base, and also often a basal conglomerate made up of diabase peb- 

 bles. 



^ An uplifted mass of impure stratified limestone, containing a fauna that indi- 

 cates its probable lower Eocene age, occurs in the diabase area between the 

 headwaters of Pescadero Creek and the San Lorenzo River. This limestone 

 appeirs to have no visible stratigraphic relations with the Miocene shales sur- 

 rounding the diabase. 



2 This basalt is the subject of a paper now in course of preparation by Prof. 

 Milnor Roberts, of the University of Washington. 



