PHYSICAL GEOGRAPHY 73 



phers, states that the Middle Miocene was probably 

 the time of the initiation of these mountains — a time 

 when the outlines of the land were more extensive 

 than now, and when the great Mid-Miocene epoch of 

 volcanism was most active. 



The present day aspects of the Coast Range High- 

 lands, however, are due to many other later events of 

 geologic history extending into the present time, which 

 have been admirably set forth in a recent paper by Mr. 

 Robin Willis,^ a son of Dr. Bailey Willis. 



According to Robin Willis, quoting Mendenhall,^ "the 

 Coast Ranges vary widely as to age." He (Robin 

 Willis) also states that the structure is of an unusual 

 type. It is controlled by the movements on the great 

 faults of the region, folding being secondary — a condi- 

 tion which the writer has long alleged to exist in 

 Southern California, and which he is glad to see so 

 ably set forth by this author. "Many of the ranges 

 are simple fault blocks but folding also has its influ- 

 ence on the broader features," which "are controlled 

 by differential uplift due to deformation, as opposed to 

 uniform uplift where the topography is controlled by 

 differential erosion". In later pages we have en- 

 deavored to say this, perhaps, in less technical lan- 

 guage.^ 



It has been the habit to carry this name "Coast 

 Ranges" southward into Southern California, but this 

 constitutes a grave error which has largely prevented 

 a proper understanding of the latter region, for no 



^Bull. Geological Society of America, Dec, 1925. 



'These allusions to the work of Mr. Robin Willis have been inserted after the 

 main body of this work was in type, and hence the omission of references to 

 them on later pages. Our only points of difference are that he includes our 

 "Transvcre Ranges" in his "Coast Range" Group. This difference is of no 

 importance at present. 



