Theory of Formation of Sedimentary Deposit. 119 



Where the rate of depression is equal to the rate of 

 supply of material, whether each is uniform, or whether 

 they vary, provided they vary by the same amount and 

 in the same sense, there will be a uniform shoreward over- 

 lap of the zones of deposition. (Figure 1.) Such an 

 equality between the two primary factors seems to have 

 existed in many localities during the periods of deposition 

 of the various sedimentary deposits as we find a gradual 

 encroachment of the limestone upon the shales and of the 

 shales upon the sandstones. 



Figure 2. 





If the rate of depression is uniform or variable but less 

 than the rate of supply of detritus, there will be a con- 

 tinuous overlap, but in the opposite sense, i.e., seaward, 

 and it may even happen that a series of land deposits 

 overlying the older marine deposits may be formed. 

 (Figure 2, ab.) These conditions are also frequently repre- 

 sented in the palaeozoic and later sediments of central 

 North America and elsewhere. In Ontario and New York 

 the conditions cited in the two previous paragraphs seem 

 to have existed until the close of the Trenton, when there 

 was a gradual shallowing of the water (accompanied pro- 

 bably by climatic changes), as indicated by the overlap of 

 the Utica shales upon the Trenton limestones. This 

 shallowing continued for a considerable length of time, so 

 that during the Medina we find there were, in certain 

 areas, broad tidal flats on which more or less arenaceous 

 deposits were laid down. The fact that the argillaceous 

 beds are separated by layers of more or less arenaceous 

 material indicates that there were slight oscillations in 

 the operation of one or other of the two primary 



