76 BULLETIN UNITED STATES GEOLOGICAL SURVEY. \Vol.vi. 



tude, at its culmination must have earned much or most of it quite beyond. To what 

 extent displaced, and how far Bnperaeded by the vegetation which in our day borders 

 •he Lee, or by ice itself, it is difficult to form mora than general conjectures — so differ- 

 ent ;in<l conflicting are the views of geologists upon the Glacial period. But upon any, 

 •r almost any, of these views, it is safe to conclude thai temperate vegetation, such 

 as preceded the refrigeration and has now again succeeded it. was either thrust out of 

 Northern Europe and the Northern Atlantic States, or was reduced to precarious ex- 

 istence and diminished forms. It also appears that, on our own continent at least, a 

 milder climate than the present, and a considerable submergence of land, transiently 

 .supervened at the north, to which The vegetation must have sensibly responded by a 

 northward movement, from which it afterward receded. 



All these vicissitudes must have left their impress upon the actual vegetation, and 

 particularly upon the trees. They furnish probable reason for the loss of American 

 types sustained by Europe. 



I conceive that three things have conspired to this loss. First, Europe, hardly ex- 

 tending south of latitude 40°, is all within the limits generally assigned to severe 

 glacial action. Second, its mountains trend east and west, from the Pyrenees to the 

 Carpathians and the Caucasus beyond, near its southern border ; and they had glaciers 

 of their own, which must have begun their operations, and poured down the north- 

 ward flanks, while the plains were still covered with forest on the retreat from the great 

 ice-wave coming from the north. Attacked both on front and rear, much of the forest 

 must have perished then and there. Third, across the line of retreat of those which 

 may have Hanked the mountain-ranges, or were stationed south of them, stretched the 

 Mediterranean, an impassable barrier. Some hardy trees may have eked out their ex- 

 istence on the uorthern shore of the Mediterranean and the Atlantic coast. But we 

 fUmbt not Taxodium and Sequoias, Magnolias and Liquidambars, and even Hickories 

 and the like were among the missing. Escape by the east, and rehabilitation from 

 that quarter until a very late period, was apparently prevented by the prolongation 

 of the Mediterranean to the Caspian and thence to the Siberiau Ocean. If we accept 

 the supposition of Nordenskiold, that anterior to the Glacial period Europe was 

 *' bounded on the south by an ocean extending from the Atlantic over the present 

 deserts of Sahara and Central Asia to the Pacific," all chance of these American types 

 having escaped from or re-eutered Europe from the south and east is excluded. 

 Europe may thus be conceived to have been for a time somewhat in the condition in 

 which Greenland is now, and indeed to have been connected with Greenland in this 

 •or in earlier times. Such a junction, cutting off access of the Gulf Stream to the polar 

 sea, would, as some think, other things remaining as they are, almost of itself give 

 glaciation to Europe. Greenland may be referred to, by way of comparison, as a 

 country which, having undergone extreme glaciation. bears the marks of it in the 

 extreme poverty of its flora, and m the absence <>t' the plants to which its southern 

 portion, extending six degrees belov* the arctic circle, might be entitled, it ought to 

 have trees and might support them. But, since destruction by glaciation, no way has 

 been open for- their return. Europe tared oraob better, but suffered in its degree in a 

 similar way. 



Turning tor a moment to the American continent for a contrast, we find the land 

 unbroken ami open down to the tropic, and the mountains miming north and south, 

 riii- trees, when touched on the north by the on-coming refrigeration, had only to 

 move their southern border southward, along an open way, as tar as the exigency 

 required : and there was no impediment to their dut' return. Then the more southern 

 latitude of the United States gave great advantage over Europe. On the Atlantic 

 border proper glaciation was felt only in the northern part, down to about latitude 

 in . in the interior <>f the country, owing doubtless to greater dryness and summer 

 heat, the limit receded greatly northward in the Mississippi Valley, and gave only 

 local glaciers to the Rocky Mountains; and no volcanic outbreaks or violent changes 

 of any kind have here occurred since the types of our present vegetation came to the 



