604 PROCEEDINGS OF THE ACADEMY OF [Sept., 



and in particular the basins, it seems probable that the great basin^ 

 was first tenanted by plants, and that the North Basin opposed this 

 migration a much larger time for the reason that this basin, which 

 presents a scene of desolation, was the seat of a local valley glacier 

 which was, perhaps, the last to disappear. As a consequence, the 

 basin presents an appearance even more xerophytic and alpine than 

 some of the upper parts of the mountain itself. The pucker bush 

 (Krummholz) reaches here an unusual development, with the trees 

 lying in most places prostrate and gnarled and twisted to a high degree. 



The swamp societies of the northern America stand in contrast to the 

 bog societies made up of more southerly forms, and must be considered 

 to be the normal hydrophytic vegetation of present climatic condi- 

 tions. The swamps of the North have had a much later origin than the 

 bogs, for we find that if the depressions provided with water have ex- 

 isted since the days of the tundra they may show a bog flora to-day; 

 if they are of recent origin the plants will correspond to the normal 

 swamp plants of the present climatic conditions. 



Another interesting problem which presents itself is that of the 

 presence of typical seashore plants on the coasts of the Great Lakes. 

 Such plants as Ammophila arundinacea Hast., Sabbatia angularis Piu'sh, 

 Lathyrus 7naritimus Bigel., Hudsonia tomentosa Nutt., Cakile americana 

 Nutt., Hibiscus moscheutos L., Gerardia purpurea L., Euphorbia polygo- 

 nifolia L., Myrica cerifera L., Strophostyles peduncularis Ell. are found 

 not only on the shores of the Great Lakes, but some of them near the 

 Lake of the Woods. The most satisfactory explanation seems to be 

 that in post-glacial times the valleys of the St. Lawrence, Hudson, Lake 

 Champlain, and probably also Lakes Ontario and Superior, were then 

 occupied by the sea, because of the northeasterly depression of the 

 land. During this period of submergence the typical seashore plants 

 gained access to the interior of the continent. 



The country south of the great ice sheet shows some interesting 

 problems of geographic distribution in line with the subject of this 

 paper. The northward extension of the pine barren flora on Long 

 Island and Staten Island is a case in point.* 



The soil of the region is generally sandy, but is occasionally more 

 firm where strata of clay approach and form the surface. The geo- 

 logical formations to the south and southeast of a line drawn from a 

 point below Long Branch to another near the head of Delaware Bay 



^ Harvey, L. H., "A Study of the Physiographic Ecology of Mount Katahdin, 

 Maine," University of Maine Studies, No. 5, December, 1903. 



^Cf. N. L. Britton, on the "Northward Extension of the N. J. Pine Barren on 

 Long and Staten Islands," Bulletin Torrey Botanical Club, VII, 81, July, 1880. 



