374 University of California Publications in Geology [Vol. 13 



This statement has been verified by Clark, 10 who has gone a step 

 farther by finding lower Cambrian fossils above the eroded granite. 

 Now, since granitic intrusions are not usually local phenomena, but, 

 on the contrary, partake of the nature of widespread revolutions, it 

 seems quite likely that the complex granitic mass of the San Bernar- 

 dino also contains rocks contemporaneous with that near Cadiz. 



At the close of Jurassic time there was a great invasion of granitic 

 rocks throughout the Sierra Nevada, and these have been traced down 

 into the San Gabriels. It therefore seems fitting to assign the latest 

 granites of the San Bernardino Mountains, herein referred to as the 

 Cactus granite, to the same age. We have seen that there are older 

 granites than these, which, however, also cut the limestone and quartz- 

 ites. Do these belong to an earlier part of the same great period of 

 intrusion or to an earlier period which, were the record complete, 

 would be found separated by denudation and sedimentation ? A post- 

 Carboniferous invasion of granites in the Sierras has been mentioned 

 by Lindgren and Turner. 20 Could it not be, then, that we have the 

 same granites represented here in the San Bernardino Mountains? 

 These are questions which naturally rise in the mind of the student 

 of west coast geology, but for their answer we can only look to the 

 future. 



The Tertiary Formations 



The Potato sandstone. — The portion of the ridge between Potato 

 Canon and Mill Creek east of Wilson Creek consists of a very hard 

 sandstone entirely different from any other rock in the district. For 

 the most part it is a well-bedded, coarse, angular arkose with angular 

 boulders of schist and granite. A yellowish variety predominates, 

 but there are also finer greenish and some red varieties. A few thin 

 beds of shale are found between the sandstone strata. The bedding 

 of the formation is somewhat variable in thickness, ranging from a 

 few inches to more than thirty feet. 



At its eastern limit the sandstone is probably in faulted contact 

 with the granite. A sharp fault contact was not seen, since the whole 

 mass is so sheared that it resembles a schist at a short distance. At 

 this point the dip is about 60° to the west, but farther west it quickly 

 flattens out and lies practically horizontal. The top of the ridge is 

 1500 feet above the bottom of Mill Creek, and, since the bottom of the 



i fl Clark, C. W., Lower and Middle Cambrian faunas from the Mohave Desert, 

 Univ. of Calif. Publ., Bull. Dept. Geol., vol. 13, no. 1, pp. 1-7. 



20 Lindgren, Waldemar, and Turner, H. W., Geologic atlas of the United 

 States, Smartsville Folio, no. 18. 



