896 W. LINDGREN—F. H. KNOWLTON 



the latter fault and the main break facing the Great Basin has 

 very pronouncedly the character of a"Horst" or "Buttress" 

 (the latter expression has been suggested by Mr. S. F. Emmons) 

 — a block standing between two sunken areas. 



This view then implies a gradual arching of the whole 

 western part of the continent and a simultaneous sinking of 

 parts of this uplifted crustal fragment, by reason of gravity, 

 along longitudinal fractures. It is in close conformity with the 

 previously announced views of King and Le Conte. 



The newly formed block of the Sierra Nevada was at once 

 attacked by the erosion ; at first probably narrow canyons were 

 cut in it, similar to those of today, though not so deep. 



Chico period ; close of the Cretaceous. — At this time the shore 

 line along the western base advanced eastward and reached at 

 least an elevation of 300 feet above the present sea level, 

 possibly considerably higher. The Chico sandstones were laid 

 down on an uneven surface, the foothills showing a relief at 

 least as prominent as those of today. This important event is 

 referred to as the Chico transgression. 



The Tejon period [Eoce?ie) . — During the Chico and the Tejon 

 the erosion was steadily at work on the flank of the Sierra 

 Nevada, the first deep canyons were widened to broad valleys in 

 the lower and middle parts of the range, while still preserving 

 their character nearer the crest. There is no evidence of any 

 Eocene deposits along the valley border in this latitude, though 

 marine Eocene occurs in the Marysville Buttes in the center of 

 the Great Valley and also further south on the east side of the 

 San Joaquin Valley. Sediments of this age may have been 

 deposited between 29*^ 30' and 38° 30', but if so, subsequent 

 erosion has removed them. An important period of erosion 

 certainly intervened between the Chico and the Miocene, and it 

 is probable that the Eocene shore line was situated somewhere 

 between the first foothills and the Marysville Buttes. This 

 erosion has removed the greater part of the Chico sandstone, so 

 that the Miocene along the valley border in part rests on older 

 rocks, in part on fragments of Chico sandstone. While remov- 



