Boreal Associations and Forests, 205 
controlled by their lime content. On these calcareous cliffs and mountains 
occur 135 Species, 94 of which are apparently unknown upon the little cal- 
careous, but strongly potassic rocks (Group I). The third group of exposures 
consists of the tableland of Mt. Albert in Gaspe, where serpentine prevails as 
a soft rock consisting of hydrated magnesium silicate. — But the appearance 
of these plants on the various mountains is a differential matter. If we 
take Mount Washington as a mountain, the summit flora is older than that 
of the lower alpine slopes of the mountains above timber-line and, the flora 
of these slopes is in turn older than that of such gorges as Tuckerman’s 
Ravine, Huntingdon Ravine and Great Gulf which probably supported local 
glaciers for many centuries after the great ice sheet had retreated from the 
Presidential range‘). Mount Katahdin, 161 miles northeast of Mount Wash- 
ington, has a less number of alpine plants than the other mountain and some 
geologists believe it to have been buried entirely beneath the glacial ice sheet, 
and the place of the boreal flora upon the retreat of the continental ice sheet 
encroached upon Mount Katahdin is determined largely by the physiography 
of the mountain. The glaciers occupying the various basins of the mountain 
retarded the vegetation of the mountain, but with a favorable opportunity the 
encroachment perhaps began from the south-west and west. This idea seems 
to be confirmed by the present distribution of the spruce and fir which ascends 
higher on this side. It seems probable that the Great Basin was the first 
tenanted on the eastern side and the North Basin opposed this migration a 
much longer 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. The area over which the bog plants spread in taking advantage 
of the numerous glacial lakes and streams must have been much more ex- 
tensive even than that now occupied by them. In the smaller glacial depres- 
sions where absence of wave action would favor littoral vegetation, the bog 
plants would become firmiy established. On the western and eastern sides of 
the glaciated area, the tundra would be closely followed by the coniferous 
forests, because of their preglacial location separated from each other by a 
treeless region. 
These conifers and those forming a still younger element surrounded the 
bog associations which were thus trapped by the environing tree vege- 
tation. As the lake filled with such plants as Poramogeton natans, P. lucens, 
Nuphar (Nymphaca) advena, Nymphaea (Castalia) odorata, fringed with a cir- 
cumarea of sedges (Dulichium arundinaceum, Scirpus, Carex) grasses (Phrag- 
mites communts) and other plants (Sarracenia purpurea, Typha latifolia, Meny- 
anthes trifoliata, Cicuta bulbifera, Scheuchzeria palustris, Utricularia (several 
species), and an outer circumarea of Cassandra (Chamaedaphne) calyculata, 
Dryopteris (Nephrodium) thelypteris, Kalmia glauca, Ledum latifolium (= L. 
ı) This assumption is traue if Mount Washington was a nunatak and not covered by ice as 
some geologists and glacialists claim. 
