Vol. XI] D1CKERSON—PT. REYES AND SANTA ROSA QUADRANGLES $47 



of the intimate relations of the marine Merced to the lavas 

 and tuff of the Sonoma volcanic series is seen. 



On top of Spring Hill, three miles west and 10 south of 

 Petaluma, the Merced sandstone is found resting upon a 

 basalt flow which is about 50 to 100 feet thick. This basalt 

 flow in turn rests upon the Franciscan, but Merced strata 

 occur at an elevation of 100 feet above sea level, one mile 

 northeast of this hill in what is clearly a sag in the old Fran- 

 ciscan surface. The lower edge of the basalt flow on Spring 

 Hill has an average altitude of about 300 to 400 feet above 

 sea level. In other words, an inequality in the erosion surface 

 of about 300 to 400 feet, is here recognized as a small outcrop 

 of Franciscan occurs at an elevation of about 50 feet in the 

 town of Petaluma, one block west of the corner of Stanley and 

 High streets. At other places in the vicinity, like peculiarities 

 are^seen in the distribution of Merced and basalt. One of con- 

 siderable local interest is found on the corner of Stanley and 

 Howard streets. (See Plate XXVIII.) Loosely consolidated 

 tan sandstones containing what are to all appearances mud balls 

 of volcanic tuff, which are from six inches to a foot in di- 

 ameter, rest upon an erosional surface of basalt. These beds 

 have a low dip of about one degree to the east. Workers 

 in making the excavations at this place reported marine 

 shells, but the writer was unable to verify this conclusion. 

 These tuffaceous sandstones, however, are areally connected 

 with tan sandstones and gravels of Reservoir Hill, elevation 

 270 feet, where casts of several pelecypods of Merced age 

 were obtained. The basalt occurring at Stanley Street is 

 relatively thin at this point, as Franciscan schists and cherts 

 occur on Stanley Street one block west of High Street, at a 

 point onlv 15 or 20 feet lower than the corner of Howard 

 and Stanley streets. One-half mile west of this locality, how- 

 ever, occurs a hill 100 feet high, consisting almost wholly of 

 basalt This same basaltic area extends southward and rises 

 to an elevation of 200 or 300 feet. From this distributional 

 study it is evident that the basalt was laid down upon an 

 irregular surface cut in the Franciscan. The local uncon- 

 formity between the Merced and basalt at the corner of Stan- 

 lev and Howard streets indicates that a part of the flows 

 actually entered the Merced Sea of that time, were partly 

 eroded, and Merced sands were in turn deposited upon them. 



