38 



University of California Publications in Geology t VoL 11 



CONCLUSIONS 



From the evidence presented, it seems reasonable to conclude that 

 the Franciscan sandstone is, in large part, a continental deposit. 

 This mode of origin has been suspected by Professor Lawson for some 

 time since in the San Francisco Folio 60 he suggested the possibility 

 that the Franciscan sandstone might be a non-marine deposit. 



The Franciscan sandstone appears to have accumulated in a region 

 sufficiently arid so that the chemical decay of rock minerals was very 

 slight. It appears to have been deposited by aggrading streams which 

 occasionally, possibly due to local uplifts, attacked and reworked por- 

 tions of the formation that had previously been deposited and con- 

 solidated. 



Certain parts of the Franciscan sandstone are undoubtedly marine. 

 The black slates at Slate's Springs are in part marine as shown by 

 their fossils. The lenses and larger bodies of radiolarian chert which 

 occur in the sandstone are unquestionably marine. These occurrences 

 are not inconsistent with the idea of continental deposition of the 

 sandstone. They simply mean that certain portions of the area must, 

 from time to time, have been invaded by the sea. This is to be ex- 

 pected since most formations recognized as continental deposits show, 

 here and there, portions which may be proved by their fossil content 

 to be marine. 61 



The presence of small coal seams favors the idea of continental 

 deposition, and is not opposed to the idea of aridity. In the river 

 bottoms where moisture was present there would be local swamps 

 where vegetation would flourish even though the general region were 

 arid. 



The absence of cross-bedding in the sandstone might seem unusual 

 on the hypothesis of continental deposition by fluviatile action. De- 

 posits laid down by ordinary streams might reasonably be expected to 

 be cross-bedded. If, however, deposition occurred from an over- 

 loaded stream carrying large quantities of feldspathic sand, no great 

 amount of sorting could occur and the absence of cross-bedding should 

 be expected. Barrell 02 states that ripple marks and current marks are 



60 San Francisco Folio, U. S. Geol. Surv. Folio no. 193, 1914. 

 «i Barrell, Jour. Geol., vol. 14, p. 354, 1906. 



62 Eecognition of Ancient Delta Deposits, Bull. Geol. Soc. Am., vol. 23, 

 p. 433, 1912. 



