GEOLOGY OF WEST COAST REGION OF UNITED STATES 



Along the west coast there is a thick and wide- 

 spread series of rocks known as the Franciscan 

 series, which, with the exception of certain gran- 

 ites and limestones that occur in the southern 

 ranges, is the oldest of the formations exposed 

 in the Coast Ranges. This series is of interest to 

 visitors of the Panama-Pacific Exposition, as it 

 forms the rock foundation of the city of San Fran- 

 cisco, as well as much of the higher and scenic 

 portions of the Coast Ranges. In spite 9f wide 

 distribution and easy access, many difficulties have 

 been encountered in the study of these rocks. Few 

 fossils have been found except tests of radiolaria 

 and foraminifera, which furnish little data suit- 

 able for geologic correlations. Close folding and 

 the general prevalence of a double cover of soil 

 and brush have hindered structural studies. One 

 of the best sections is found in Corral Hollow, 

 southwest of the old coal mining camp of Tesla, 

 and can be reached by automobile from San Fran- 

 cisco. Here a measured section includes 15,000 

 feet of sedimentary rock without exposing either 

 the top or the bottom of the series. The lowest 

 formation exposed is a dense blue sandstone cut 

 by innumerable intersecting quartz veinlets. Above 

 are the Corral Hollow shales which contain massive 

 beds of crumpled and folded cherts, and, especially 

 in the vicinity of serpentine intrusions, of law- 

 sonite, chlorite, and glaucophane bearing schists, 

 which seem to be peculiar to the Franciscan series 

 and similar metamorphics in Greece, Asia Minor, 

 and Japan. The upper member is the slightly meta- 

 morphosed Oakridge sandstone. 



The geologists who have studied the Franciscan 

 formation are not in agreement as to its age. It 

 has been assigned in part to the Cretaceous; but 

 its position under the Knoxville series (Early Cre- 

 taceous) suggests a pre-Cretaceous age. A portion 

 of the series lies unconformably on granite, and if 

 this granite is contemporaneous with the last of 

 the granites of the Sierra Nevada, a post-Jurassic 

 age is indicated; but the history of the batholith 

 is as yet undeciphered in detail, and possibly may 

 have occupied several geological periods in the 

 forming. 



We conclude, therefore, that an important 

 trough, in which deposition was active, was located 

 west of the Sierra Nevada Mountains. On the east 

 side of this trough the Mariposa formation (Jurassic 

 slates) was deposited, and the Franciscan series to 

 the west. This trough was probably post-Palaeozoic 

 and pre-Cretaceous in age. To the west the more 



45 



