240 



University of California Publications in Geology [Vol. 11 



THE FRANCISCAN GROUP 



Rocks of the Franciscan group are found throughout the Cali- 

 fornia Coast Ranges, extending from the Santa Ynez Mountains 

 northward into Humboldt County. Further north they appear again 

 in the southern end of the Oregon Coast Ranges and also in the 

 Olympic Mountains of Washington. The total extent of the Fran- 

 ciscan terrane along the Pacific Coast is, therefore, in the neighbor- 

 hood of 1000 miles. 



Over this great extent it shows a striking uniformity. It com- 

 prises several unusual rock types the peculiarities of which, even to 

 minute details, persist throughout. 



The sedimentary portion comprises arkose sandstones, radiolarian 

 cherts and foraminiferal limestones with subordinate shale and con- 

 glomerate. Estimates of its thickness range from 5000 to 20,000 feet. 



The sandstone, which makes up the greater part of the sediment- 

 ary portion, is a massive, firmly cemented rock, breaking like quartz- 

 ite through the grains as well as through the cement. It is pale 

 bluish to greenish in color when fresh, but disintegrates easily when 

 exposed to the action of the weather. The constituent grains are very 

 angular and comprise notable amounts of feldspar and ferromag- 

 nesian minerals. The sandstone contains small lenses of conglomerate 

 and there is some admixture of shale. Fossils are extremely rare and 

 the formation appears to have been, in large part, a continental 

 deposit laid down by aggrading streams in an arid region. 1 



The limestone formation (Calera limestone) is of limited thick- 

 ness, being sixty feet thick on the San Francisco Peninsula. Lime- 

 stone may occur in lenses in the sandstone. It is usually white to 

 gray in color, but is sometimes dark and fetid. It contains numerous 

 foraminiferal shells. There is considerable chert in the limestone in 

 the form of nodules, patchy lenses and blanket-like masses. 



The strata are cut by numerous masses of basic igneous rocks of 

 several types. Basalts and diabases, showing an ellipsoidal or pillow 

 structure are common. These are unquestionably intrusive into the 

 Franciscan sediments at many places in the California Coast Ranges. 

 At other localities pillow basalts show no evidence of intrusive action 

 and represent contemporaneous lava flows. Associated with the pil- 

 low basalts, are various basic rocks without pillow structure. These 



i Davis, E. F., The Franciscan Sandstone, IJniv. Calif. Publ., Bull. Dept. Geol., 

 vol. 11, pp. 1-14, 1918. 



