Vol. XI] DICKERSOX—PT. REYES AND SANTA ROSA QUADRANGLES 597 



thickness and rest upon the relatively impervious rocks of 

 the Franciscan group. The pore space in these sandstones 

 is large and hence much water can be reservoired within 

 them. If it were not for this fact the numerous small chicken 

 ranches around Petaluma on the north and west, as well as 

 around Penn Grove, would be failures, owing to the lack of 

 water. Nearly all these places have shallow wells sunk in 

 the Merced sandstone and can obtain a moderate but inde- 

 pendent supply of water. The light sandy soil yielded by the 

 Merced is warmed readily by the spring sun and, with aid of 

 fertilizer, produces fair yields of green crops necessary for 

 the fowls. A third factor in making Petaluma a poultry 

 center is due to cheap transportation provided by small river 

 steamers which navigate Petaluma Creek, a stream drowned 

 by the subsidence of the Bay Region. 



The ground-water relations of the Merced are splendidly 

 exemplified in Spring Hill, a half mile west of Marin School, 

 about three miles west of Petaluma town. (See Geological 

 Cross-section along the line A-B, Petaluma and Santa Rosa 

 Quadrangles.) As will be seen from this section the de- 

 posits of Merced rest upon basalt, which in turn rests upon 

 Franciscan rocks. This relationship is nicely shown in Plate 

 XXVIII, Figure 1. The Merced sandstone resting upon this 

 basaltic flow is from 50 to 100 feet in thickness, and all around 

 Spring Hill at the contact between the Merced and the basalt 

 numerous springs occur. The dwellers on Spring Hill are 

 thus favored by Nature, who, not satisfied with the imper- 

 vious Franciscan beds, laid down upon them an excellent 

 grouting of hard basalt, 50 to 100 feet in thickness, and then 

 the waves of the Pliocene sea deposited coarse grained Mer- 

 ced sands, thus providing a sponge-like mass for the favored 

 residents of this beautiful hill country. The dwellers within 

 Franciscan areas in this region must be content with obtain- 

 ing a small water supply from the shallow alluvium of small 

 valleys, or pipe it from Sonoma Mountain. 



The city of Petaluma has its water reservoir on the south- 

 western slope of Sonoma Mountain in the Santa Rosa Quad- 

 rangle, directly within the Hayward Rift. Although the 

 writer did not verify this condition, it seems probable they 



