162 N. S. SHALER — EVIDENCES AS TO CHANGE OF SEALEVEL. 



occur along the shoreline extending from the Lesser Antilles southward 

 are of the reentrant type — that is, they have been formed in the upper 

 parts of flooded valleys. 



The flooding of the Amazon and the La Plata rivers constitutes the 

 most extensive indentations of their kind in the world, while a number 

 of minor streams afford similar though less important evidence of the 

 same nature. I am aware that the absence of a normal delta in the 

 valley of the Amazon has, to the satisfaction of some naturalists, been 

 explained by the action of the incoming tidal wave moving in the form 

 of an " eagre " or " bore " up the pathway of that stream. I am inclined 

 to think that the importance of the work done by this irregular occa- 

 sional wave has been much overestimated. Moreover, the form of valley, 

 with its widely separated bluffs, as is the case with the lower portion of 

 the Mississippi, clearly shows that the basin has recently been lowered 

 to a considerable depth. 



On the western shore of South America, as on the northern continent, 

 the evidence concerning the present higher level of the sea is obscure as 

 compared with that which has been obtained along the Atlantic coast. 



In the Patagonian section the maps of the fiords appear to indicate the 

 higher level of the shore at the time when the topography of the country 

 was made ; yet it should not be overlooked that these valleys now'occu- 

 pied by oceanic water may possibly be due to the cutting action of the 

 ice. 



The shoreline extending from Patagonia northward to the isthmus of 

 Darien will have to be omitted from the discussion, as I have been 

 unable to find any evidence indicating the flooding of valleys which is 

 worth considering. 



Africa. — The continent of Africa, so far as we may determine by the 

 imperfect maps of its shoreline, affords less evidence of valley-flooding 

 than any other of the great lands. It is also conspicuous for the lack of 

 those islands near the main shore which often afford indications of recent 

 subsidence. 



So far as I have been able to find, data in the way of soundings to 

 show the form and extent of the continental submarine shelf are not yet 

 in shape for discussion. I am inclined to think that this feature exists 

 there, but under the circumstances indicated have no opinion to offer 

 as to its value. As a whole, the geographic and geologic indications 

 which we have concerning Africa south of the Sahara lead us to suppose 

 that the land has maintained its position with reference to the sea in a 

 more permanent manner than has any other continent bordering on the 

 Atlantic. 



Within and north of the Sahara there appears to have been in Tertiary 



