﻿150 GEOLOGICAL ACTION OF 



Without designing to undertake any thing like a minute description of geological 

 districts, I will proceed in a very brief and general manner to cite a few instances of 

 the geological action of the tides in the past ages of the earth's history. The instances 

 will be taken from the tertiary and subsequent periods, principally for the obvious 

 reason that they have undergone fewer changes since the time of their deposition. 

 The idea immediately suggests itself, that, if this theory of aqueous deposit be correct, 

 we ought to find the alluvial, quaternary, and tertiary formations in close proximity to 

 each other, particularly where those of an earlier period are so situated as to preclude 

 the possibility of any great alteration in the transmission of the tide-waves during that 

 long interval. Taking the first example from this country, it will be seen by a reference 

 o Sir Charles Lyell's geological map of the United States, that the cretaceous, tertiary, 

 and post-pliocene deposits lie along the borders of the coast, succeeding each other 

 outwards in the order in which they are named. They appear to follow one another 

 in regular course, according to the laws of tidal deposit, each period showing larger than 

 that which precedes it, and which it overlies. 



The post-pliocene, being outside of all, and accumulated on the shores of the Northern 

 and Eastern States by the constant transporting power of the current of the flood, effect- 

 ed a gradual change in the progress of the tide-wave, diverting it more to the eastward. 

 Long Island, and the islands on the southern coast of Massachusetts, all of which were 

 the shoal formations of that period, contributed greatly towards this change. The local- 

 ity of these islands was determined by the nucleus of a former period, and their form or 

 outline must be ascribed to the action of tidal currents. In the case of Long Island,* 

 the shoal commenced probably at the heights of Brooklyn, the highest part of the island, 

 and was gradually built up towards the east. The deposit was at first assisted by the 

 confluence of the tidal with the river currents ; and afterwards by that of the tides of 

 the Sound conflicting with those of the open sea, the former running at that time, as now, 

 through the deep and rocky chasm of Hell-Gate. 



The islands on the Massachusetts coast and the district of Cape Cod were con- 

 structed by a similar process, their forms also being due to the different directions and 

 modes of action of the currents in that region. But as the eastern coast is the most 

 exposed to the destructive action of the prevailing storms, its shores, being gradually 

 acted upon and degraded as the continent was upheaved, now appear as precipitous 

 cliffs, where the stratified deposits show to the greatest advantage. 



* In the minute and able account of the geology of this island, particularly of the alluvial formations and 

 their changes, by Professor Mather, the reader will find several cases in which this theory of the geological 

 action of the tidal currents is applicable. 



