1918] 



Davis: The Franciscan Sandstone 



9 



In the Sierra Nevada, the rocks of the Upper Cretaceous Chico 

 formation are found resting on the eroded surface of the granite. 

 This granite has been intruded into the older rocks of the region ; the 

 youngest group that it has cut, is the Jurassic Mariposa formation. 

 The Mariposa slates contain a fauna that has been regarded as late 

 Jurassic, since it includes several ammonites which are characteristic 

 of the late Jurassic all over the world. 18 



These facts limit the period of the intrusion of the Sierra Nevada 

 granite to the interval between the end of the Jurassic and the Upper 

 Cretaceous. A considerable erosion interval, during which the granite 

 was revealed, must have elapsed between the intrusion and the deposi- 

 tion of the Chico. It is generally assumed that the intrusion of this 

 granite and the folding of the rocks of the basement complex of the 

 Sierra Nevada occurred during the widespread earth movements that 

 are known to have followed the close of the Jurassic throughout west- 

 ern North America. During early and middle Cretaceous the region 

 which is now the Sierra Nevada was probably undergoing erosion. 

 On the basis of the foregoing facts and assumptions regarding the 

 age of the Sierra Nevada granite, and assuming the contemporaneous 

 intrusion of the Coast Range granite, it would follow that the Fran- 

 ciscan formation must be post-Jurassic. As before stated, its relation 

 to the Knoxville suggests that it must be pre-Cretaceous. 



If this reasoning be correct it means that in the interval of time 

 between the close of the Jurassic and the beginning of the Cretaceous 

 there occurred : 



1. The intrusion of the granite, with the probable production of a 

 mountain range near the site of the present Coast Ranges. 



2. The erosion of the region and the exposure of the granite. 



3. The deposition of Franciscan sediments to a thickness of from 

 one to four miles. 



4. The intrusion of these sediments by basic igneous rocks and 

 their disturbance by crustal movements. 



5. Their erosion in pre-Knoxville time. 



All these occurrences took place before the beginning of the Lower 

 Cretaceous as now recognized. As Professor Lawson has pointed out, 

 this complexity of events would seem to require the extension of the 

 geological time scale at the interval between the recognized Cretaceous 

 and the Jurassic. 



18 Stanton cited by Diller in Bull. Geol. Soc. Am., vol. 19, p. 399, 1908. 



