UNIVERSITY OF CALIFORNIA. 

 Bulletin of the Department of Geology. 

 Vol. 1, No. 3, pp. 71-1 14, Pis. 6-7. ANDREW C. LAWSON, Editor. 



T H E 



ERUPTIVE ROCKS OF POINT BONITA 



BY 



F. Leslie Ransome, 

 Fellow in the University of California. 



CONTENTS. 



Introduction 71 



General Account of the Geology 73 



The Spheroidal Basalt 75 



Occurrence and Structural Peculiarities 75 



Microscopic Petrography 80 



The Diabase 85 



Occurrence 85 



Microscopic Petrography 88 



Variolitic Facies 99 



The Pyroclastic Rocks 103 



The Relations of the Eruptive Rocks to Each Other 104 



Analyses of Eruptive Rocks of Point Bonita 106 



The Sedimentary Formations 106 



Character and Occurrence 106 



Relations to the Eruptive Rocks 108 



The Origin of the Spheroidal Structure no 



INTRODUCTION. 



Point Bonita forms the southern extremity of Marin County, 

 California, being the most westerly headland on the north shore 

 of the entrance to San Francisco Bay. In passing out through the 

 Golden Gate, the coast-line is here seen to turn abruptly to the 

 north, and it is along this stretch of shore for a distance, in a 

 straight line, of about four-fifths of a mile, that the rocks to be 

 described are mainly exposed. This portion of the coast has been 

 eroded by the waves of the Pacific into precipitous cliffs, loftiest at 

 the point indicated on the map as the site of the old tower, dimin- 

 ishing in height towards the north, and ceasing at the sand-bar, 

 which, in ordinary weather, separates the waters of the ocean from 



