39° 



University of California. 



[Vol. i. 



gressive subsidence associated with heavy accumulations of sedi- 

 ment. The persistency of a similar association between subsidence 

 and sedimentation has long been recognized, and two diametrically 

 opposed views have been, and still are, held in regard to it. 

 According to the one, long- continued and heavy deposition can 

 only take place concurrently with, and in consequence of, subsi- 

 dence of the area receiving the sediment, and elevation of the area 

 denuded. It is this movement which disturbs the previously exist- 

 ing equilibrium and sets in motion the forces of erosion, transporta- 

 tion and deposition. Those holding this view may or may not com- 

 mit themselves to any discussion of the rigidity of the earth's crust, 

 but the greater number of them assume that it is possessed of suffi- 

 cient rigidity to be unaffected by the transference of material from 

 one area to another by the ordinary processes of sedimentary 

 transportation. In contrast with this older view, which is simply a 

 condensed expression of the first principles of dynamic geology as 

 taught by Lyell, there has of late years been advanced by very able 

 geologists, particularly in America, the so-called "doctrine" of 

 isostasy, in accordance with which loading by sedimentation is 

 looked upon as the active cause of subsidence, and lightening by 

 erosion as the immediate cause of elevation. The avidity with 

 which this doctrine has been accepted in the United States, and to 

 a less extent in Britain, taken in connection with the slight amount of 

 effective protest from those holding the other view, is rather remark- 

 able; and we are treated to the somewhat unusual spectacle of two 

 opposite, although not necessarily wholly contradictory, theories, 

 existing amicably side by side. Before proceeding to the applica- 

 tion of either of these opposing theories to the particular case under 

 consideration, it will be well to consider in a general way the devel- 

 opment of the theory of isostasy as applied to areas of subsidence 

 receiving sediment. 



DEVELOPMENT OF THE THEORY OF ISOSTASY. 



With the impetus given by Lyell to the inductive method in 

 geological investigation, it was not long before various observers 

 noted the now well-established fact, that great deposition of sedi- 

 ment has been attended by a more or less gradual subsidence. 



