Glacial Changes in Christiania. 325 



from slightly above to slightly below the sea. The beds are often 

 brought to light in the excavations for buildings in the lower part 

 of the city of Christiaaia. The most characteristic species is 

 Scrohicularia piperata, a southern form which extends to the Medi- 

 terranean. Though still living off the west coast of Norway at 

 depths of 0-8 meti'es, it no longer inhabits the Christiania fjord. 



The total number of species in this youngest post-Glacial deposit, 

 including the Scrobicularia-clays, is 245, of which 34 are Arctic, 

 102 boreal, and 109 Lusitanian species, or approximately iu the 

 proportions of y, f, f- The Lusitanian or southern element has 

 distinctly increased in these beds, both by the introduction of new 

 species and the predominance of others. 



The living molluscan fauna of the Christiania fjord comprises 268 

 species, as against 255 species in the post-Glacial fauna; 210 species 

 are common to both. Forty-five post-Glacial species have not as yet 

 been met with living in the Christiania fjord, whilst 53 species now 

 living in the fjord are not known in the post-Glacial beds of the 

 region. A significant number of the Lusitanian forms of the post- 

 Glacial beds have disappeared from the fjord and the nearest portions 

 of the Norwegian coast, even after the land had been elevated to its 

 present level. Amongst those in the existing fauna and not known 

 in the post-Glacial deposits, there are several Ai'ctic species which 

 apparently have migrated to Scandinavia from the northern part 

 of the American continent, where they still live ; they are unknown 

 in the Polar Sea to the north of Asia. This shifting of species 

 shows that the boreal element is more prominent in the present 

 fauna of the fjord than during the latest post-Glacial time, when the 

 climate also seems to have been 2° C. warmer than at present. 



There is no conclusive evidence in the Christiania district of 

 a depression corresponding to the Ancylus or Littorina depression 

 of the Baltic basin. In some localities a deposit of warm, com- 

 paratively shallow-water, post-Glacial clay (Isocardia-clay as a rule) 

 rests unconformably, with a sharply defined junction-plane, on the 

 cold deep-water Area-clay, from the upper surface of which boulders 

 project to which balani and oysters are attached, indicating that 

 this surface cannot have been elevated and exposed to atmospheric 

 erosion before the post-Glacial clay was laid down over it. Some 

 of the recorded occurrences of clays of deep-water origin intervening 

 between shallow-water shelly sands may have originated by the 

 sliding down of clay -beds from higher levels of valley slopes, 

 instances of which are not uncommon. 



At the end of the book a table is given showing the changes of 

 level, the variations in the molluscan fauna, and in the climate of the 

 Christiania region from the Yoldia-clay of the ra period to the 

 present, and this is followed by a list of species of all the known 

 shell-bearing mollusca in the late Glacial and post-Glacial deposits, 

 showing their distribution in the different periods. 



In concluding this notice we desire to call attention to the 

 admirable way in which the mollusca, so fully treated in the work, 



