The relationship between the palaearctic and nearctic faunas 273 



eustatic movements of the sea have therefore played at most a quite inferior role in 

 its formation. 



(b) Isostatic movements are local and due to ice-pressure during periods of 

 glaciation. In the central part of an inland ice they may amount to more than 200 

 meters, for instance in Scandinavia. If Greenland were cleared of ice, it would 

 undoubtedly rise to a considerable degree but, as we have learned from investiga- 

 tions in Scandinavia, the "isobase o", the line of equilibrium, with no postglacial 

 upheaval and, consequently, no previous glacial down-pressure, runs close to the 

 former ice-margin, or even inside it. The rise would affect only the immediately 

 surrounding sea-bottom. 



An increase of the Greenlandic inland-ice, on the other hand, and a contempor- 

 aneous ice-cap over Iceland, would possibly cause a slight upheaval of the interjac- 

 ent sea-bottom but involve very little isostatic effect on the almost three times as 

 long sea-bed between Iceland and Scotland. The proposed land-bridge could not 

 arise, or even become considerably favoured, by isostatic movements. 



(c) The possible influence of tectonic movements and volcanic activity remains 

 to be considered. 



Iceland is a country with still vividly living volcanic powers. In comparatively 

 recent time, geologically speaking, these have been active within a much wider 

 area. Basalt of the same kind as that which built up the frame of the Icelandic 

 plateau occurs in parts of East Greenland, on the Faeroes, on some of the Inner 

 Hebrides, and in northeastern Ireland. Certain fossiliferous layers of this system 

 in Iceland, the Faeroes and Ireland have been regarded as homologous and con- 

 temperaneous. It is generally assumed that these now widely separated basalt 

 occurrences are only remnants of a former continuous land, formed through vol- 

 canic eruptions in Tertiary time, from Eocene and onwards. Soon after its rise, 

 this mighty basalt plateau became subject to violent tectonic movements and 

 eventually was broken down to present conditions. 



Geologists apparently are not able to answer the question most important to a 

 biogeographer: at what time was the connection broken and the present isolation 

 of these islands definite? According to Thoroddsen (1914), Iceland was already a 

 separate island at the beginning of the Pliocene. Later geological students have 

 thought that Thoroddsen generally overestimated the age of Icelandic geological 

 formations but the current view seems to presume that the Tertiary transatlantic 

 basalt isthmus was broken down almost to a definite extent before Pleistocene 

 time.^ 



^ Dr. Sigurdur Thorarinsson, the wellknown Icelandic geologist, informs me, however, 

 {in litt.) that to his mind no definite facts seem to argue against the possibility that the Ter- 

 tiary land-bridge persisted into part of the Pleistocene period. 



18 — 565597 Lindroth 



