148 N. S. SHALER — EVIDENCES AS TO CHANGE OP SEALEVEL. 



share in the expressions and structure of the surface and underlying 

 rocks which has been brought about by the several oscillations of the 

 sea. It is to this interpretation that we shall now give our attention. 



Accepting the fact, which is abundantly proved by the study of shore- 

 lines and by evidence which is from day to day increasing in amount 

 and definite value, that seashores are subjected to frequent oscillations of 

 level, we may first take into account the point that the lesser changes of 

 level are much more frequent than the greater, or, in other words, that 

 along any coastal belt the improbability of the shoreline having within a 

 certain time been at any horizontal plane increases with its vertical de- 

 parture from the present sealevel. Inasmuch, however, as the lands are 

 prevailingly rising and the sea-basins deepening in the manner before 

 noted, the likelihood that the sea within a given time has had its plane 

 at a given height above the present position is greater than that it has 

 been at a given depth below that level. 



Another statement of this proposition may be made as follows : On any 

 coast the probability that the shoreline has been farther out to sea than 

 it now is is less than the probability that the line has been farther inland 

 in the measure to which elevating forces, that serve to maintain the land 

 against the erosion to which it is constantly subjected, have acted. 



Results achieved by deforming Agencies. — These considerations serve to 

 show us that the farther we go above the sealevel the less likely it is that 

 we shall find definitely ascertainable marks of marine action. The 

 probability that slight oscillations exceed those of great amount, and the 

 likelihood that the lands will continue to rise, together with the rapid 

 waj-in which the erosive agents of the atmosphere attack the sea-made 

 topography, tend to destroy the evidence of ancient marine action. 



The normal result of the above described actions is to develop next 

 the shore, both to the seaward and landward, a system of erosion and 

 construction planes all sloping toward deep water. Where the process 

 is long continued there may be relatively little diff'erence in form ex- 

 hibited by this surface, either below or above the water. There are, 

 however, in all cases certain distinctions between destruction and con- 

 struction planes. The former usually retain at least a substantial shadow 

 of their drainage system, while the latter show when they have been 

 elevated, and less distinctly, by soundings, a peculiar undulating topog- 

 raphy, such as is produced by submarine conditions, but never by the 

 land waters. 



Moiuitain-bailding. — If continental shores had the simple history indi- 

 cated by the statements made above the interpretation of coastline changes 

 would be much simpler than it is. In most cases, however, a number of 

 perturbing influences enter into the action, of which the formation of 



