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



eastern side of the continent to the northward the signs indicating recent 

 downward movement appear to me evident and to indicate a progressive 

 subsidence of a somewhat uniform nature to near the pole. Reentrant 

 valleys begin to be indicated along the Texan shore. At the Mississippi 

 we find proof that this great valley has recently been loAvered, so that 

 the sea penetrated far into the land, perhaps to and even beyond the 

 junction of the main stream with the Ohio. In Mobile bay, as well as 

 in one or two reentrants to the eastward along the Gulf shore, there are 

 similar evidences of subsidence. 



Floridian Peninsula. — In the peninsula of Florida we have a geographic/ 

 feature which throws much light in diverse ways on the general history 

 of the Atlantic coastline. I have elsewhere noted* that Florida appears 

 to be a submarine fold, the summit of which is at the present time ele- 

 vated to less than one-tenth of its total height above the surface of the 

 sea. There are reasons, however, for believing that the whole peninsula 

 has recently stood much higher than at present. The evidence on this 

 point is as follows : 



In the first place, the coastline exhibits a number of flooded valleys, 

 of Avhich that of the Saint Johns river is the best preserved and most 

 conspicuous. There are several other vales which recently were em- 

 bayments of the sea that have of late become effaced by detrital deposits 

 and swamp accumulations. Some of these channels along the west coast, 

 which are now completely filled by a sand plain formed in a very late 

 minor oscillation that reduced the peninsula to very small dimensions, 

 are evidently of considerable depth, as is shown by excavations made to 

 obtain the phosphate nodules which were washed into them by marine 

 action. It is tolerably evident, indeed, that if the recent deposits here 

 alluded to were removed the surface of the Cretaceous and Tertiar}^ beds 

 would be found deeply scarred by gorge-like valleys. 



Another evidence indicating the recent elevation of Florida to a con- 

 siderable height above the sea is found in the fact that on the eastern 

 and western shores of the northern half of the peninsula there come 

 forth from a considerable depth beneath the sea great springs of fresh 

 water. One of them, nearly off Saint Augustine, is formed by the. dis- 

 charge of such a volume of water that where it rises through the heavier 

 marine fluid it forms a slight elevation, from which the outsetting stream 

 is so strong that, according to trustworthy reports made to me, a boat has 

 to be rowed with some energy to attain the center of the disk. As there 

 is no way in which we can account for the excavation of the subterra- 

 nean channels through which these waters course to their present exits be- 

 neath the sea except by the supposition that they were made as caverns 



*See Bulletin of this Society, vol. 5, p. 206. 



