18 



University of California. 



[Vol. 3, 



by finding' these terraces cut on undoubted Upper Pliocene 

 strata which had been tilted and much eroded before the sub- 

 mergence to which the terraces are due, so he placed the upper 

 terrace early in the Quaternary era although recognizing the 

 comparative recency of the lower ones. Fairbanks was fortunate 

 in encountering the terraces eroded into tilted, very late Pliocene 

 strata and he discovered that a period of land elevation must 

 have occurred between the subsidences to which are due, respect- 

 ively, the Upper Pliocene deposits and the terraces. More 

 recently, Lawson has attacked the subject from the same stand- 

 point as the writer, namely, by observing the degree of preserva- 

 tion of the terraces and now is in substantial accord with the 

 present writer in placing all of those of the San Pedro region 

 very late in the Quaternary era.* 



Near the head of Mint Canon, about six miles north of Lang 

 Station, there is a basin several miles wide. Its floor is a dis- 

 sected detrital slope, built up at the base of the Sierra Pelona 

 schist mountain and declining toward the opposite side of the 

 basin. Granite and highly tilted, cemented Tertiary breccia- 

 conglomerate were planed off evenly and capped by a bed of 

 partly water worn schist and quartz from the Sierra Pelona, fif- 

 teen to fifty feet thick and apparently spread over the basin floor 

 by torrential floods during abnormally heavy but very localized 

 rains. Uplift and possibly some tilting have caused the erosion 

 in the floor of the basin of very narrow canons 100 to 300 feet 

 deep, by which the detrital slope is cnt up into mesa-like rem- 

 nants. The rock excavated was chiefly soft semi-decomposed 

 granite and the canons do not indicate an age for the alluvial 

 capping of the mesas greater than the Red Bluff epoch. I 

 correlate the detrital slope with the 400-foot terrace of the 

 neighboring Soledad Canon as their dissection was evidently due 

 to the same disturbance. 



It may be well at this stage of the discussion to notice the 

 fact that the erosion of canons into and beneath the detrital 

 slopes does not necessarily imply an uplift of the area, as it is 

 well known that an increased and better distributed rainfall 



* Communicated. 



