204 Part III. Chapter 3. 
since the close of the glacial period. If their deductions are sound, then 
certain elements of the flora of the northern part of America cannot be 
older than 25,000 years at the outside. Some of its elements may be 
much older and we have reason to believe that many boreal plants existed 
as such on the nunataks which were unglaciated areas above the sea of ice. 
With the renewal of a milder climate and the consequent recession of the 
glaciers, the plant associations would gradually spread in the direction of con- 
tinous habitats and generally northward. Several waves of plant migration 
may be recognized. The glacial element would follow the retreating edge 
of the ice to be followed by bog and tundra types which would then push 
into the barren ground left by the retreating ice. Many glacial plants, existing 
in the extremely fine and mixed soils, apparently rich in the availability of 
potassium, calcium and often magnesium.derived from rocks of different kinds, 
would be left as relicts on such mountain peaks as Mt. Marcy, Mt. Washington 
and Mount Katahdin, or in circumscribed areas in the lowlands, such as the 
botanist finds in New Brunswick and Labrador. The determination of the 
postglacial habitat of each species of arctic-alpine plant was regulated by the. 
soil preferences of that species. Whether the plant was to grow upon a soil 
derived from rocks containing potash, calcium or magnesium was a matter of 
specific preference. FERNALD (I. c.) divides the known localities where arctic- 
alpine plants are found into three main groups, corresponding with the cha- 
racter of the rocks from which the soils are derived. In the first group of 
alpine areas, he includes the White Mountains, the Adirondack Mountains, the 
highest summits of the Green Mountains of Vermont; Baldpate, Abraham, 
Saddleback, Bigelow, Katahdin and nearly all the other naked-topped mountains 
of Maine; the great table land of Table-top Mountain in Gaspe; and Mt. Desert 
Island and other exposed parts of the eastern coast of Maine. The predominant 
rocks are granite, or gneiss, and all are especially high in potassium which 
becomes the distinctive soil element of the alpine areas constituting our first 
group, where we find ı22 plants, nearly two thirds of which in their alpine 
distribution are quite unknown on the mountains of the other groups. Such 
plants are Aerochlos alpına, Salix phylicifolia, S. argyrocarpa, Cassiope hyp- 
noides, Arenaria groenlandica etc. The second group of alpine areas includes 
the cliffs of Perce, of the north coast of the Gaspe Peninsula, and of Bic, 
which are chiefly limestones, calcareous sandstones, limestone conglomerates, 
and calcareous slates; the northwestern escarpments of Table-top Mountain, 
where Salix vestita, S. glauca, Saxifraga oppositifolia, S. Aizoon, S. aizoides 
and Primula mistassinica abound are limestones; the river cliffs and ledges 
of many streams of eastern Quebec, northern New Brunswick and Maine 
characterized by Asplenium veride, Woodsia alpina, W. glabella, Carex eburnea, 
Tofieldia glutinosa, Astragalus elegans, Hedysarum boreale (= 4; americanum), 
Shepher.dia canadensis, Primula mistassinica, Pinguicula vulgaris, Erigeron 
kyssopifolius are chiefly limestones or limy slates; the famous cliffs at Willough- 
by are of impure limestone, and the soil of the cliffs in Smugglers Notch are 
VAL en Ara SEI er Kr 
