1904.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 29 



for fifty feet and again entered the shale. After passing through a 

 few hundred feet of the shale the well entered the diabase again, 

 and was still in the igneous rock when discontinued. Microscopic 

 slides of the diabase encountered in this well showed it to be very 

 similar to the exposure a quarter of a mile to the east, except that 

 the rock was badly blackened with carbonaceous matter. 



The Tuff. 



The tuffs associated with the diabase are confined to the Langley 

 Hill-Mindego Hill igneous area, of which they form the major por- 

 tion. Within this area are also found diabase both of the diabasic 

 and basaltic types, limestone beds, limestone dikes or intrusions, 

 shale and sandstone. It is to be regretted that all of the rocks 

 within this area cannot be differentiated on the map, as their areal 

 distribution would throw much light on the structure of the terri- 

 tory within which they occur. Beds of sandstone containing a 

 typical lower Miocene fauna (given on a previous page) are found 

 between layers of the tuff, while the shales containing Pecten peck- 

 hamiy when associated with the tuffs, are always found above them. 

 This places most of the tuffs in the lower Miocene, with a possibil- 

 ity of their extending into the middle Miocene. Layers of one of 

 the basaltic facies of the diabase are found in such relation to the 

 tuff as would indicate the contemporaneity of the two. This theory 

 is strengthened by the fact that this characteristic basaltic facies, 

 with the exception of the outflow near Stanford University, has 

 been found so far only within the Langley Hill-Mindego Hill igne- 

 ous area, to which the tuff is confined. The true diabase is later 

 than the basaltic facies and associated tuffs, as it is intrusive both in 

 the tuffs and in shale beds overlying them. The Purisima formation 

 overlies unconformably both the tuffs and their overlying shale 

 beds. (See Fig. 3.) 



The tuffs vary in composition from solid masses of basaltic dia- 

 base fragments to almost pure limestone, sandstone and shale, de- 

 pending on the conditions under which they were formed. It is a 

 significant fact that the fragments of igneous rock in the tuff are, 

 in all cases so far noticed, composed of the basaltic facies of the 

 diabase. This is to be expected, as the extrusive forms of the rock 

 would naturally be finer grained than the intrusive ones. The ma- 

 terial in which the fragments of igneous rock are imbedded is gen- 

 erally more or less limy, thus showing that the fragments were de- 



