160 Rhodora [JULY 
outlier of a southern Coastal Plain genus, fruit profusely among the 
tussocks of Scirpus caespitosus and other Arctic species on the highest 
alpine barrens of Newfoundland; and Potamogeton Oakesianus is 
there as thoroughly at home with Sparganiwm hyperboreum of Green- 
land as on Nantucket! with Najas guadalupensis. 
The withdrawal from the ocean of a great body of water to form the 
Pleistocene glaciers and the gradual restoration of it to cause the 
submergence of our coastal bench give us, then, a sufficient basis upon 
which to build the land bridge necessary to carry the Coastal Plain 
plants, the muskrat and the vole to Newfoundland, and gradually 
to isolate them upon that island. But other factors may have had 
to do with the former height of the coastal bench. For instance, in 
his paper On Pleistocene Changes of Level in eastern North America, 
Baron Gerard de Geer * shows that although in the later Pleistocene 
submergence the ocean covered areas which now are far above sea-level 
(Mt. Desert Island with late Pleistocene beaches at 210 feet on the hills; 
St. John, New Brunswick, at about 269 feet; Dalhousie, New Bruns- 
wick, at 174 feet; Riviére du Loup at 373 feet; with points farther in- 
land showing beaches at 600-800 feet), southeast and east of this now 
uplifted region there is a belt including Perth Amboy (New Jersey), 
Long Island, Martha’s Vineyard, southeastern Massachusetts, south- 
ern, central and eastern Nova Scotia, eastern Prince Edward Island, 
the Magdalen Islands, and the southern margin of Newfoundland, 
which shows no sign of Post-Pleistocene uplift. This line (de Geer’s 
isobase for zero) south of which no evidence has been found of recent 
elevation is so nearly coincident with the northern line of our supposed 
land-bridge that the eastern portion of de Geer’s map is here repro- 
duced (plate 91, fig. 13), with the omission of the colors and some 
inland features which do not bear directly on our problem. And, 
although inland from de Geer’s isobase for zero the land has clearly 
been elevated since Glacial times,’ Professor J. B. Woodworth, who 
has kindly helped me to several references, informs me that he knows 
no evidence of Post-Glacial elevation south of that isobase but that, 
whatever may be the changes at present going on, there are many eVi- 
1"It [Potamogeton Oakesianus] is oe Peteomene in some of the small ponds 
of Nantucket, where it fruits very freely. ng, Mem. Torr. Bot. Cl. iii. No- 
De Geer, Proc. Bost. Soc. Nat. Hist. xxv. 454-477 (1892). . 
* Recent observations have altered in some details the courses of de Geer’s 
isobases but have not changed his zero line along the coast west of Newfoundland. 
