Geology. 357 



the late glacial and post glacial bogs ; detailed accounts, both 

 geologic and ecologic, of the Danish bogs ; lists of animals and 

 plants found ; and an account of the time of immigration and the 

 subsequent history in Denmark of a large number of trees and 

 shrubs. 



The pollen-statistical method is largely used, and frequency 

 curves are constructed for the different species. The results 

 appear to point to the validity of the so-called Blytt-Sernander 

 hypothesis of past alternations of climate. This study starts with 

 late glacial times, considered, on the basis of the geological work 

 of de Geer and Liden, to have been about 11,000 B. C. or slightly 

 earlier. This was the time of the Older Dry as flora of Dry as 

 octopetala, Salix polaris, Salix reticulata, Bettila nana, etc., indi- 

 cating a subarctic climate in Denmark with July temperatures of 

 8°-12° c. This was followed by the AUerod period marked by 

 the introduction of Betula intermedia, Betula puhescens, Juni- 

 perus communis, Finns silvestris, Populus tremula, etc. indi- 

 cating a temperate continental climate with July temperatures of 

 12° -15° C. The Allerod period was followed by the Younger 

 Dry as period with a recurrence of the climate and flora of the 

 Older Dryas period. Following this w^as a long warm period 

 estimated as having lasted for about 7,000 years, commencing 

 about 7500 B. C, during which the climate was warmer than it is 

 at the present time. This warm period, which corresponds to the 

 Ancylus Lake and the Litorina sea in the history of the Baltic, 

 is divided into 1. An older MuUerup, Pine, or Boreal period, 

 during Avhich the climate was dry and rather warm, with such 

 plants as Alnus glutinosa, Tilia cordata, Ulmits glabra, Corniis 

 sanguinea, Corylus avellana. Primus padus, Finns silvestris, etc. : 

 and 2. A mixed oak forest or Atlantic period at which time the 

 climate was warm and humid, with July "temperatures of about 

 17° C, and with Acer platanoides, Fraxius excelsior, Humulus 

 lupidus, Trapa natans and, toward its close, Fogus silvatica. 

 Then followed 3 : The beginning of the Beech period, about 

 contemporaneous with the Bronze age, when July temperatures 

 reached 18° C. and the climate was again dry and warm. At 

 about 400 B. C. the temperature lessened and the climate became 

 more humid. This corresponds to the Limnaea Sea stage of the 

 Baltic, or the Iron Age in Denmark, and is known as the sub- 

 Atlantic period. This continued to the beginning of the His- 

 toric period which in Denmark was about 800 A. D. e. w. b. 



8. The Study of Geological Maps; by Gertrude L. Elles. 

 Pp. 74, with 7 plates, 1 map, and 64 figures. Cambridge Geologi- 

 cal Series, Cambridge University Press, 1921. — The subject mat- 

 ter of this book was assembled by the author to make it available 

 to students of geology. One of the chief aims of the book is to 

 train students to think in three dimensions, and for this purpose 



Am. Jour. Sci.— Fifth Series, Vol. II. No. 12. — December, 1921. 

 25 



