86 ON EROSIONS OF THE EARTH'S SURFACE. 



syenite, while around Boston, they are softer metamorphic slates, we cannot doubt 

 why Boston Harbor has been scooped out by the action of tides and breakers ; and 

 probably we would extend the same conclusion to the whole of Massachusetts Bay; 

 although at present Cape Cod is extending in a northeasterly direction, because a 

 current sets in that direction along the coast. In passing from Cape Ann to the Bay 

 of Fundy, some 300 or 400 miles, we find almost the whole coast serrated by 

 fiords, some of them 20 miles long, including the many islands that once constituted 

 continuous ridges. 



2. In the Extensive Denudations of the Strata by Oceanic Agency, when the Surface 

 of Continents sunk beneath, and emerged from, the Waters. 



Tbis has doubtless been the most powerful of all the agencies of erosion which 

 the surface has undergone. In South Wales, where the geology has been examined 

 with almost unequalled thoroughness and accuracy, by the Ordnance Geological 

 Surveyors, Professor Ramsay has made it almost certain, that as much as 10,000 

 feet in perpendicular thickness has disappeared. I do not think Geological obser- 

 vations in this country have been prosecuted with the minute accuracy requisite 

 to determine the denudation here. From what I have observed, however, it would 

 not seem extravagant to assert, that an equal amount of strata have disappeared 

 from some parts of our country. In my paper on Terraces, I have endeavored to 

 show, that since the tertiary period, the continent has once sunk below the ocean, 

 and once emerged from it. Furthermore, I intend in this paper to point out 

 certain valleys that must have been occupied by rivers before the continent's last 

 submergence. Tracing back its history still further, we may be sure that during 

 the deposition of the coal measures, a large portion of it at least must have been 

 below the waters. Yet previously, or perhaps contemporaneously, large portions 

 of the surface must have been dry land, to nourish the prolific flora which pro- 

 duced the coal. During the Devonian and Silurian periods, we have still clearer 

 evidence that almost the entire continent was covered by the ocean. 



We may then be nearly or quite sure of at least three depressions of the North 

 American continent, beneath, and an equal number of elevations above the ocean, 

 since the fossiliferous rocks began to be formed. And, in general, it is clear that 

 these vertical movements were but slightly paroxysmal; so that every part of 

 the surface has been again and again exposed to the long-continued action of 

 waves, tides, and currents. The amount of erosion must have been prodigiously 

 great ; and, in my opinion, we find the evidence of it almost everywhere in the 

 mountainous districts of our country. Even where our valleys are so narrow at 

 their lower part, that rivers may well have worn them out, their upper part is so 

 widened that only waves and tides, rushing back and forth between rocky islands, 

 could have caused it. Indeed, the ragged isolated appearance of a large part of 

 our mountains, save on their northern sides, can be explained only on the supposi- 

 tion of having been subject to powerful oceanic action. 



Sir Charles Lyell, in connection with Professor John Locke, has made an esti- 

 mate of the amount of denudation of the rocks above the lower Silurian at Cincin- 



