404 University of California "Publications in Geology [Vol. 11 



Franciscan at the close of the period. Further it is not always easy 

 to decide whether or not a certain mass of basic igneous rock is in- 

 trusive or extrusive. The commonly used criterion of pillow structure 

 is useless. It has often beeu assumed that the presence of a pillow 

 structure was proof of a submarine extrusive origin. As has been 

 shown by Lawson and Ransome, the presence of pillow structure has 

 no such significance. On Angel Island, Ransome found an unmistak- 

 able intrusion of ellipsoidal fourchite, cutting sandstones and cherts. 

 Professor Lawson has described similar sections at Hunter's Point and 

 in Marin County. There are numerous other places in the Coast 

 Ranges where pillow lavas show intrusive contacts. The grain of 

 the pillow lavas, also, is no criterion of their mode of eruption. While 

 certain pillow lavas may be rather coarse grained, most of them show 

 an exceedingly compact texture, and this is characteristic of some 

 intrusive masses of considerable size. One has to resort to other 

 means to determine the nature of such rocks. 



Professor Lawson has described intercalated flows of basic lavas 

 in the lower sandstone formation of the Franciscan group. At Point 

 Bonita, Ransome found evidence that there were both intrusive and 

 extrusive varieties of pillow lava. The writer has seen several cases of 

 basic lavas, some of which showed ellipsoidal structure, intercalated 

 with the radiolarian cherts in such a way as to leave no question of 

 their extrusive nature. At the Miller Ranch, near Sargent, there is 

 an area of several square miles of fragmental igneous rock of Fran- 

 ciscan age, which is lithologically identical with the type of lava 

 that elsewhere shows the ellipsoidal structure. It is poorly sorted 

 and shows no evidence of stratification, so that it may possibly repre- 

 sent a tuff erupted beneath the sea. 



In view of the evidence it is clear that all during Franciscan 

 time there were eruptions of basic igneous rock of a peculiar type. 

 During these progressive eruptions, the earlier formed rocks, such as 

 sandstones and cherts, would be intruded and metamorphosed by 

 masses of igneous rocks passing through them on their way to the 

 surface. Both intrusive and extrusive facies were produced. 



In California, the intrusive masses of these igneous rocks show 

 contact zones which are characterized by the addition of soda to the 

 altered rocks. In other regions igneous rocks of the same type as the 

 igneous rocks of the Franciscan, have produced contact zones char- 

 acterized by the addition of soda. Probably, though not certainly, 

 this soda was in the form of sodium silicate. 



