Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 54} 



and vicinity. In this area, while no continuous section oc- 

 curs, we find several fair exposures sufficient to enable us 

 to distinguish this formation from the Merced group which 

 occurs in the near vicinity. A fine-grained, light-brown 

 sandstone occurs at Eureka School at the cross roads. The 

 bedding in this massive sandstone makes the structure obscure, 

 but the approximate strike is N. 60° W., and dip 25° N. A 

 coarser brown sandstone and its associated conglomerates 

 which are free from basaltic or andesitic pebbles or other 

 fragments of the Sonoma group are found along the road 

 at intervals of about a mile southwest of Eureka School. 

 The absence of these rocks from the Petaluma formation, 

 and their general presence in the sandstone of the Merced 

 group, enable one to discriminate these formations in the field. 

 The pebbles in these conglomerates consist of chert, quartz, 

 schist and sandstone fragments and most of them have been 

 derived from the Franciscan group. Along the road which 

 runs northwesterly from Eureka School in the bottom of the 

 stream just west of the road, green clay with thin strata of 

 interbedded limestone occurs. This clay is well stratified but 

 does not exhibit characteristic shale structure in most ex- 

 posures. Limestone is frequently present in the form of small 

 nodular masses and is very argillaceous. Resting upon this 

 green clay is a tan sandstone containing green grains. These 

 beds have a strike of N. 55° W., and dip of 20° S., while 

 just a short distance on the north side of the road reverse 

 dips were found in the same material in the small gulches 

 entering the major stream. The green and light gray clays 

 are well exposed in a tributary of Tolay Creek about 2 

 miles southeast of Eureka School. These deposits are char- 

 acteristically lacustrine. That this is true, is amply proved 

 by the presence of brackish and freshwater fossils in them in 

 the extension of this formation to the northwest. Certain 

 other lithologic variations are best observed here as well. A 

 sandstone which occurs one-quarter mile east of Waugh 

 School, Santa Rosa Quadrangle, is interbedded with clays 

 which yield a stiff adobe soil. This sandstone contains ob- 

 scure fossils — plant remains — and it is lighter in color than 

 the prevailing sandstone of the Merced group. Small chunks 

 of light colored clay ranging from a quarter inch to an inch 



