4o8 



University of California. 



[Vol. i. 



ing the floor of the Great Valley could have remained utterly- 

 unmoved by the vigorous orogenic forces at work on both sides of 

 it. The natural question is, How was it affected? It was almost 

 certainly bent into a shallow syncline- — substantially the structure 

 which it has at the present day. Had it originally been buckled 

 upward into an anticline by the pressure applied to its edges, it is 

 difficult to see how it could have later been changed into a syncline, 

 since the orogenic movements have been progressively at work, 

 and the valley floor consequently under a compressive strain all the 

 time. The fact that the Coast Ranges and Sierra Nevada sweep 

 around the southern end of the Great Valley, and join near latitude 

 35°, and that a similar junction of the bounding ranges terminates 

 it on the north, also points to an original synclinal structure. For 

 near where the ranges unite, the valley floor must certainly on the 

 whole have been synclinally depressed, and these lines of weakness 

 initiated at both ends of a long, narrow crust-block under lateral 

 compressive strain, would be very likely to propagate them- 

 selves toward the middle portion of the valley, so as to give it its 

 present shallow, basin-like form. It thus appears that there 

 is no necessity to call upon the hypothesis of isostasy in order to 

 account for the present form and condition of the Great Valley, but 

 that, on the contrary, the latter presents no features which can not 

 be explained through the activity of geologic processes less hypo- 

 thetical in character than is the theory of isostasy. 



Facts Opposed to Isostatic Subsidence. — That the floor of the 

 Great Valley is not perfectly isostatic is sufficiently shown by the 

 way in which the Sacramento, San Joaquin, and other of the larger 

 streams, flow through the plain upon low ridges of their own con- 

 struction. Similar evidence of a stronger kind is afforded by the 

 alluvial fans which have been projected into the valley by the 

 streams flowing down from the Sierra Nevada. Such a fan has 

 been built up within the last fifty years by the Yuba River, near 

 Marysville, whereby the latter town, formerly above, has now 

 become below the river at high water. But by far the best exam- 

 ples of these structures are those which have been pushed out on 

 the eastern side of the San Joaquin and Tulare Valleys by the San 

 Joaquin, Kings, and Kaweah Rivers, and, further south, by the Kern 



