TRANSGRESSIVE OVERLAP 725 



With uniform rates of both subsidence and supply, three cases 

 may be considered : 



a. Rate of depression is equal to the rate of supply of 



detritus. 



b. Rate of depression exceeds rate of supply. 



c. Rate of depression is exceeded by rate of supply. 



The first case would result in the production of relatively stationary 

 conditions, if the shore-line were bounded by a vertical face, when 

 a uniform regular amount of detritus equaling the amount of sub- 

 sidence would produce a constant depth of water. With a shelving 

 shore, on the other hand, a uniform regular transgressive move- 

 ment would occur, with a regular and uniform change in the char- 

 acter of the deposit at any given point. The second case will pro- 

 duce a rapid transgressional movement with a less normal suc- 

 cession of formations, while the third will produce either stationary 

 or retreating coast-line, coupled with an increasing amount of 

 subaerial deposition. 



I. Transgressive Movements. 



a. Rate of Depression Equals Rate of Supply. Under these 

 conditions a uniform and progressive overlapping of each later 

 layer over all the preceding ones takes place. Each layer has a 

 rudaceous or coarsely arenaceous texture at the shore, and grades 

 seaward into finer arenaceous and ultimately lutaceous material. 

 If the shore is composed of old crystalline rocks, the rudytes and 

 arenytes resulting from their destruction wall be largely siliceous, 

 while the lutytes will be argillaceous and more or less micaceous. 

 The coarse shoreward ends of the beds, when viewed in their 

 ensemble, will appear as a single coarse bed resting upon the old 

 land. From the consideration of its origin, however, it will be 

 seen that no two portions of the bed along a line transverse to 

 the seashore will be of the same age, each seaward portion will 

 be younger than that lying next to it nearer the land. Thus the 

 formation line, limiting the basal conglomerate or sandstone, will 

 run diagonally upward through the planes of synchronous deposi- 

 tion. The basal bed is not generally a conglomerate, for where 

 the sea transgresses upon an old land which has long been subject 

 to subaerial disintegration a basal sandstone will be produced, since 

 there is not coarse material enough to form pebbles. When de- 

 composition has gone on for a long time prior to the transgression 

 of the sea, and when the decomposed material is subjected to a 



