724 PRINCIPLES OF STRATIGRAPHY 



Three relations of land and sea may be recognized : 



1. Subsiding land block or rising sea-level. 



2. Stationary sea-level. 



3. Rising land block or falling sea-level. 



Each of these conditions is further complicated by variation in 

 the rate of subsidence, or elevation, and the rate of supply of detri- 

 tal material. In general, rising sea-level produces transgressive 

 movements, except where the supply of detritus is excessive, when 

 stationary or regressive movements are produced. Stationary and 

 falling sea-level produce regressive movements. 



I. Rising Sea-Level or Positive Diastrophic Movement. 



A subsiding land block or rising sea-level may be either of local 

 or of world-wide effect. Its cause may be diastrophic movements, or 

 the gentle displacement of the water by the accumulation of sedi- 

 ment on the ocean floor. Such rising of the sea-level produces a con- 

 tinuous transgression of the sea upon the land, i. c, a landward mi- 

 gration of the shore-line. The rate of migration, other things being 

 equal, varies inversely as the steepness of the slope of the coast. 

 Thus a slight depression of a very gentle shore will cause a great 

 transgression of the sea, while even a considerable depression of a 

 steep or vertical coast may produce little or no transgression. In the 

 following discussions, the slope of the land surface affected by the 

 transgression will be considered as a gentle one, such as is pro- 

 duced by a period of prolonged erosion of an old land surface. 



Wilson (g:ii8; Rutot-7) has tabulated the possible relationship 

 between the rate of depression of the land (rise of the sea'-level) 

 and the rate of supply of detritus, as follows : 



Rate of /•_■> Rate of 



Depression Uniform -r:;; 1^| j-^ Uniform supply of 



"■^^ ^'^ detritus 



t 



Variable -^ \^\ -^Variable 



The simplest conditions are those in which the rate of depression 

 and the rate of supply are both uniform. These alone will be con- 

 sidered; the variable conditions of either one, or of both factors, 

 will produce corresponding variations of the norm to an almost 

 unlimited degree. 



