Vol. 4] Osmont. — Geological Section of Coast Ranges. 4!) 



It was not found practicable to work out in detail the struc- 

 ture of this formation, though with good maps and careful work 

 it probably could be done. The whole series has been displayed 

 in the sections as a unit. 



Areal Distribution. — As regards its distribution within the 

 territory under discussion, it is found in one small and four 

 large areas. The most westerly of these forms a large part of 

 the high ridge, following the coast line from Mt. Tamalpais 

 northward through Marin County to the Russian River, and 

 northward, except that part occupied by the granitics previously 

 described. This Franciscan area varies in width from eight to 

 fifteen miles, and forms a high coastal ridge, with a very even 

 crest line, at an elevation of 1,000 to 1,200 feet above sea level. 

 Being for the most part composed of rather hard rocks which 

 resist erosion well, the canon topography is frequently rugged, 

 and the hillsides usually dotted with clumps of gray boulders 

 of hard sandstone or chert, while trees seem to grow better upon 

 the soil furnished by these rocks than upon that of the later 

 formations. A trained eye can easily distinguish at a distance 

 this formation from the smoother, more rounded contours and 

 treeless aspect of the overlying Tertiary sedimentaries. 



The second Franciscan area is a narrow strip about two 

 miles wide, which forms the core of the range of hills between 

 Santa Rosa and Napa valleys. Its extension north of a point 

 about two miles north of the Petrified Forest, where section AB 

 crosses, is not known to the writer, but it is found in the hills 

 to the east of Rincon Valley, near Santa Rosa, but does not 

 appear at the surface in section CD between Petaluma and Napa, 

 it being covered over by Tertiary sediments and volcanics. At 

 the waterworks, two miles east of Napa, a well sunk through the 

 andesite encountered Franciscan chert at a depth of 1,500 feet. 



The above mentioned area is worthy of note, since there is 

 evidence to show that in the vicinity of the Petrified Forest at 

 least, dry land existed during the period of volcanic disturbance, 

 which will be shown presently inaugurated the later Pliocene 

 Section AB passes through this area, crossing it at a point about 

 1,300 feet above sea-level. Large pieces of petrified wood lie 

 strewn along the flanks of this ridge where the pumicious tuff 



