THE DISTRIBUTION OF PLANTS IN NEW HAMPSHIRE. 393 



show that they could never have migrated to these stations under ordi- 

 nary causes. The science of geology has led to the probable solution of 

 this problem. It has been found necessary, for the explanation of many 

 phenomena in the surface geology of the northern temperate zone, to sup- 

 pose that at a comparatively recent geological period the climatic condi- 

 tions were wholly different from those of the present time. The ruins of 

 a burned building do not tell their story more plainly than do the bould- 

 ers of our hills and the worn and striated sides of our mountains prove 

 the existence of glaciers and icebergs among them at no very distant 

 date in geological history. The explanation which this affords of the 

 origin of an arctic flora upon high mountains in the temperate zone, has 

 been pointed out by one of the foremost theorists of the present day. 

 As the low temperature of the frigid zone became gradually extended 

 over this whole area, the forms of vegetation peculiar to an arctic climate 

 took the place of those which had previously existed, while these receded 

 to the south. Again, upon the gradual return of a more genial climate 

 throughout this area, the arctic flora disappeared, following the retreat of 

 the causes by which it was brought, and only remaining, with the rees- 

 tablishment of warmth and fertility, upon those higher mountain summits 

 whose elevation renders them arctic islands in the middle of the temper- 

 ate zone. He who ascends to this altitude has a similar opportunity for 

 botanic study as if he made a journey to the north, passing first from the 

 noble forests, with which we are familiar, to those of stunted growth, and, 

 finally leaving them behind altogether, at length arriving at the barren 

 and bleak regions beneath the Arctic Circle. 



In approaching these mountain summits, one is first struck by the 

 appearance of the firs and spruces, which gradually become more and 

 more dwarfish, at length rising but a few feet from the ground, the 

 branches spreading out horizontally many feet, and becoming thickly 

 interwoven. These present a comparatively even upper surface, which is 

 often firm enough to walk upon. At length these disappear wholly, and 

 give place to the Lapland rhododendron, Labrador tea, dwarf birch, and 

 alpine willows, all of which, after rising a few inches above the ground, 

 spread out over the surface of the nearest rock, thereby gaining warmth, 

 which enables them to exist in spite of tempest and cold. These in their 



turn give place to the Greenland sandwort, the diapensia, the cassiope, 

 VOL. i. 52 



