346 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



temperature continued to rise were joined b}" oak, Xorway maple, 

 and especially hazel. Southern plants reached their maximum 

 northern distribution, and Andersson has plotted tliis on maps in 

 the case of the hazel and the water Caltrop {Trapa natans). The 

 summer and autumn temperatures during this period were about 

 2|° C. above the present, but the ivy, which is limited by the winter 

 isotherm of — 3|° C, extended no farther north than at present, indi- 

 cating that there was no appreciable change in the winter tempera- 

 tures. 



Andersson finds that the passage from the Ancylus lake to the 

 Litorma sea took place at about the maximum of temperature, but 

 very soon afterwards the climate began to deteriorate, and as a result 

 of the examination of the moraines of recent glaciers formed during 

 the last centuries, Andersson concluded that a slow secular deteriora- 

 tion of temperature is probably still taking place. He thus finds 

 the warm period to end at an earlier period than does Sernander. 

 Further, he will not admit the alternation of wet and dry periods 

 postulated by Blytt and Sernander. The result of the Lltorina 

 subsidence was to convert the warm, dry period into a warm, wet 

 period ; as the land rose again the climate gradually became colder 

 and drier until it reached its present conditions. The flora associ- 

 ated with the Litorina subsidence suggests an annual precipitation 

 in south Sweden of about 1,000 millimeters. The tree stools of 

 Blytt's " subboreal " layer Andersson considers to occur at all levels 

 in the peat, and to be really the result of the bog rising above its 

 water table. 



Sernander supports his conclusions by reference to stools in lake 

 basins, and to calcareous tufa deposits. Gavelin (111) studied the 

 changes of level in Lakes Vanstern and Kalfven in Smaland, and 

 found that there were two periods, which he attributes to the boreal 

 and subboreal periods, during which the climate was so dry that 

 trees were able to grow below the level of the outlet of the lakes, so 

 that they must have been without outlet during the greater part of 

 the year. The calcareous tufa of Skultorp in Vastergotland,' studied 

 by Hulth (112) bears this out, for intercalated in the tufa are two 

 mold beds during which it ceased to form and soil accumulated on 

 its surface. 



Altogether, the evidence seems to favor Sernander's interpreta- 

 tion far more than Andersson's. 



In Norway the sequence of events has been described by J. Holm- 

 boe and P. A. 0yen (113). In marine terraces of the maximum 

 late glacial depression, associated with recession moraines on the 

 Norwegian coast, Rekstad (111), 0yen, and Kolderup have found 

 arctic plants, especially Salix polarh. As the land rose the climate 

 became markedly milder, and in a marine bed underlying a peat bog 

 145 meters above sea level have been found Betula odorafa and Juni- 



