VARIATIONS OF SEA LEVEL. 33 



of ordinary mountains, but rather through the opportunity which these 

 diversely shaped and irregularly disposed basins have afforded for the varied 

 application of the stresses. 



As before noted, the evidence derived from the geological history of 

 southeastern Massachusetts and the neighboring portions of the shore to, 

 the northward as far as the Gulf of St. Lawrence, and to the southward 

 into the Carolinas, shows that while this coast line has been subjected to 

 repeated and considerable variations of level, it manifests an equally clear 

 tendency to return to about its original position. Beginning with the 

 Cambrian time, we find reason to believe that this region was coastal at the 

 outset of the Olenellus epoch. If the rocks of the Roxbury conglomerate 

 be of the Potsdam period, the same was true at the last of the Cambrian 

 stages. It is again the case in the Carboniferous, in the Trias, the lower 

 Cretaceous, the middle Tertiary, and at the present day. Evidence found on 

 the coast of Maine indicates that the coast in that part of the field was also 

 near by in the Devonian period. Thus, in eight or nine of the great periods, 

 well spaced through recorded geological time, we find the coast of this dis- 

 trict near to its present attitude. As the action of erosion during the periods 

 of elevation, and the accidental burial beneath later deposits of portions of 

 the strata, are likely to have obliterated much of the record, the point which 

 we are endeavoring to make appears to be well affirmed. This point is of 

 evident value in the present inquiry, for it serves to show a reason why 

 extensive erosion valleys are characteristic of the Atlantic coast. In those 

 phases of the coastal movement in which the land has been above the 

 present level of the sea, there has been an opportunity for the formation of 

 extensive valleys of erosion, which, from time to time, with the downsinking 

 of the shore line as a whole and the downward warping, concomitant with 

 the extensive deposition, have had a chance to take on their present peculiar 

 character. 



In this connection it should be noted that the usual tendency of shore- 

 line changes on the periphery of the continental fold is to return the coast 

 after each considerable oscillation to somewhere near where it was before. 

 Elsewhere, more than once, I have called attention to the facts that within 

 the ordinary growth of the great corrugations on the earth's surface the 

 movements are normalty those of downsinking of the ocean floors and 

 uprising of the emerged portions of the continental mass, and that this 



MON XXXIII 3 



