1904.] NATURAL SCIENCES OF PHILADELPHIA. 603 



ing tree vegetation, and as the bog was gradually transformed by bio- 

 logic influences into firmer ground gradually encroached on the bog 

 plant associations. Present bog habitats are continuations of similar 

 habitats which existed in early postglacial times, when tundra 

 conditions and tundra vegetation were dominant.^ The fourth 

 element just mentioned consisted of deciduous shrubs and trees, 

 oaks, hickories and the like, which at present are south of the 

 great coniferous belt of forest. In the East, among the highlands, 

 exceptional circumstances were afforded for the preservation of the 

 northern forms. 



The flora of Mount Washington is perhaps an exception to this. 

 During the glacial period it was a nunatak, and during this time it was 

 tenanted by such plants as Sile^ie acaulis L., Arenaria groenlandica 

 Sprang., Geum radiahnn Michx., Solidago virga-aurea L. var. alpina 

 Bigel., Prenanthes bootii Gray, Cassiope hypnoides Don, Bryanthus 

 taxifolius Gray, Diapensia lapponica L., Oxyria digyna Hill, Salix phy- 

 licifolia L., Salix uva-ursi Pursh, Salix herbacea L., Phleum alpinum 

 L., Lycopodium selago L., etc., which have remained as permanent 

 tenants of this mountain. If w^e take Mount Washington as a 

 mountain, the summit flora is older than that of the lower alpine 

 slopes of the mountain above timber-line, and the flora, therefore, of 

 these slopes is in turn older than that of such gorges as Tuckerman's 

 Ravine, Huntingdon Ravine and Great Gulf, which probably sup- 

 ported local glaciers for many centuries after the great ice sheet had 

 retreated from the Presidential Range. 



Mount Katahdin, 161 miles northeast of Mount Washington, has a 

 less number of alpine plants than that mountain, and most geologists 

 believe it to have been buried entirely beneath the glacial^ ice sheet. 

 If that is so, then the alpine flora of Mount Katahdin is, as a floristic 

 element, much younger in point of time than that of Mount Washing- 

 ton. The same differential arrangement of the plants on Mount 

 Katahdin is found as on Mount Washington. The place where 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 re vegetation of the mountain, but with a 

 favorable opportunity the encroachment perhaps began from the 

 southwest and west. This idea seems to be confirmed by the present 

 distribution of the spruce and fir which ascend higher on this side 

 and their apparently greater age. As to the east side of the mountain, 



^'Transeau, E. N., Botanical Gazette, XXXVI: 401. 



