348 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



The replacement of the oak by the beech was due rather to a de- 

 crease in the summer warmth than to a change in the humidity; 

 this decrease in temperature appears to be still in progress, for the 

 beech, though thriving in the south of the country, has difficulty in 

 maintaining its ground in the north. 



Regarding the postglacial period in north Germany, a great 

 amount of evidence was presented at the International Geological 

 Congress at Stockholm in 1910 by Herren Schulz, Krause, Ramann, 

 Weber, Stoller, Graebner, Menzel, and Wahnschaffe (115). The 

 views of these authors are very conflicting. The prevailing note is 

 one of caution in the handling of peat bog sections, as there is a 

 natural sequence of events in the growth of a bog, quite apart from 

 climatic changes. The normal section of a north German bog ac- 

 cording to C. A. Weber is : 



1. Glacial floor. 



2. Lacustrine deposits. 



3. Sedge pent. Telm&tlc. 



4. Brushwood peat (Alnctum). Serniterrestrial. 



5. Fir and birch wood peat above generally a layer of fir stools, below 



one or two stool layers. Terrestrial. 



6. Older Sphagnum peat. Semiterrestrial. 



7. " Grenzhorizont," a hardened surface with heath peat. Terrestrial. 



8. Younger Sphagnum peat. Semiterrestrial. 



In the first six of these beds he finds nothing which is not the di- 

 rect result of the growth of the peat bog, but the layer of heath peat, 

 No. 7, can not be so explained; it certainly indicates a very marked 

 and long dry period intercalated between two wet periods. The 

 " Grenztorf " is a widespread horizon, which according to Weber 

 falls at the end of the neolithic, long after the Litorina period. Ra- 

 mann, however, considered that even the Grenztorf is a conse- 

 quence of the Sphagnum peat outgrowing its water supply. 



J. Stoller, as a result of studies of pollen grains in the peat bogs, 

 also regards the Grentztorf as representing a dry period, during 

 which the oak predominated, but he places it at the end of the An- 

 cylus and the beginning of Litorina time. The melting time of the 

 ice was dry and fairly cold, the remainder of postglacial time was 

 moist. Gradmann also assumed two dry periods, one late glacial, 

 and the other in the neolithic period. The latter was marked by 

 the Grenztorf , loess younger than Penck's Daunstadium (see Switz- 

 erland) and steppe mammals, especially large numbers of wild 

 horses. The researches of Schulz on the origin and present distribu- 

 tion of the plants of north Germany also point in the same direc- 

 tion, and suggest that at one period the summers were warmer than 

 and the winters as warm as the present. 



Probably the most valuable of the contributions was that by H. 

 Menzel, dealing with the land and fresh-water Mollusca. He 



