22 



HAEHL AND ARNOLD — THE MIOCENE DIABASE. 



[Feb. 5, 



Fig. 3. North-south section from Mindego Hill to Tar Creek, showing the 

 relation of the diabase tuffs to the overlying Purisima beds. 



Pliocene. — The Fio-isima Formation. Within the area under 

 discussion is an extensive series of conglomerates, fine-grained 

 sandstones and shales for which the writers propose the name " Pur- 

 isima formation.''^ This name has been chosen because of the 

 typical development of the formation in the vicinity of Purisima 

 Creek, San Mateo County. The Purisima beds lie unconformably 

 upon the Vaquero sandstone and Monterey shale, and at the top 

 grade into beds having a fauna somewhat similar to that of the 

 Merced formation. Its upper limit may be defined as the base of 

 the Merced. In age the Purisima probably represents the lower, 

 and perhaps middle. Pliocene. The individuality of the fauna, 

 stratigraphy and lithology of this formation appears to warrant the 

 application of a new and distinctive name. Fig. 4 shows a typical 

 section of the Purisima formation in the area. 



Puri'iima. 5hev.les ond-S.S. . -soo f<?et 



(j<re.zr\ 6a.r)d5to/ie 5 • 



Con^lom ei'Ate • 20 



Lower ana MiaaieMrcre.Te lioo .. 



Fig. 4. A typical section in the diabase area. 



Stratigraphically the Purisima formation presents a uniform cross 



1 The junior author now has in course of preparation a paper giving in detail 

 the characteristics and faunal relations of this formation. 



