92 



NATURE 



[September 15, .^21 



for raised beaches about 25 ft. above the 

 present level with a warm fauna extended from 

 the east of North America to Spitsbergen and the 

 White Sea. This suggests some general factor 

 which by altering the circulation of the North 

 Atlantic piled up its waters in cold temperate and 

 polar latitudes, producing a period of widespread 

 development of warm maritime climates termed 

 the " Climatic Optimum." 



By 3000 B.C. another wave of elevation affected 

 the southern half of the British Isles and neigh- 

 bouring parts of Europe, which stood about 90 ft. 

 above their present level. At the same time 

 the climate became drier, and a magnificent 

 growth of forests occurred even on the bogs of 

 Ireland, which were extensively inhabited by 

 Neolithic man. Many shallow lakes were more 

 or less dried up and trees grew on their floors 

 (Fig. 2); this is the Later Forest Phase. Tree 

 stools in exposed situations in the west of Ire- 

 land, as well as evidence of considerable sea-borne 



Fig 2. — Submerged tree stools in lough, Ireland. 



commerce between Scandinavia and the British 

 Isles, testify to an absence of strong winds. Con- 

 ditions were anticyclonic, with possibly cold win- 

 ters, but with fine, warm summers and a relatively 

 small rainfall. Neolithic civilisation rose rapidly 

 to its culminating point, especially in naturally 

 moist countries like Norway and Ireland. This 

 period, in fact, appears to coincide with the 

 legendary Heroic age of Ireland, when the vigour 

 of the Irish reached a level they have never since 

 attained, an interesting confirmation of Ellsworth 

 Huntington's theory that at present the high 

 humidity of that country lowers the energy of its 

 inhabitants. 



By 1600 B.C. the land had again sunk to its pre- 

 sent level, or possibly a few feet lower. The 

 favourable anticyclonic conditions gave place to 

 great storminess with relatively heavy rain- 

 fall, and there set in a period of in- 

 tense peat formation in Ireland, Scotland, 



NO. 2707, VOL. 108] 



Scandinavia, and North Germany. This 

 growth went on even over high ground which had 

 not previously been covered by peat, for in Ire- 

 land tumuli of the Bronze age are found resting 

 on rock and covered by several feet of bog. As 

 the Bronze age gave place to the Early Iron age 

 the climate of. North-west Europe became very un- 

 favourable, and the submergence of the early civi- 

 lisation is described in Norse sagas and Germanic 

 myths — the " Twilight of the Gods," when frost 

 and snow ruled the world for generations (about 

 650 to 400 B.C.). Peat formation went on even 

 over the Frisian dunes. 



This wet period, whatever its cause, was wide- 

 spread, as is shown in Huntington's curves of 

 tree-growth in California and climate in western 

 Asia ; the same author also believes that the Medi- 

 terranean lands had a heavier rainfall from about 

 500 B.C. to 200 A.D., and the quietude of central 

 Asia at this time suggests similar conditions there. 

 It seems that the phase was marked by a general 

 increase of the storminess and 

 rainfall of the temperate regions 

 of the northern hemisphere at 

 least, with a maximum between 

 Ireland and North Germany, in- 

 dicating probably that the Baltic 

 again became the favourite track 

 of depressions from the Atlantic. 

 The Peat-bog Phase passed into 

 the Present Phase fairly abruptly 

 about 300 A.D. 



An interesting astronomical 

 theory put forward a few years 

 ago by O. Pettersson provides a 

 possible explanation of the stormy 

 climate of the Peat-bog Phase, 

 which reached a maximum about 

 400 B.C. Without going into de- 

 tails, his theory is that storminess 

 in the North Atlantic depends on 

 the juxtaposition of masses of 

 warm and cold water and on the 

 presence of much ice ; this is 

 favoured by increased strength of 

 the tides. The " tide-generating force " 

 passes through a series of cycUc fluctua- 

 tions, and according to his calculations 

 reached maxima about 3500 B.C., 2100 B.C., 

 350 B.C., and 1434 A.D. That of 2500 b.c. 

 comes into, and may have contributed to, the Mari- 

 time Phase; that of 2100 b.c. falls near the 

 middle of the Later Forest Phase, and has left no 

 trace; but that of 350 b.c. coincides accurately 

 with the period of maximum storminess of the 

 Peat-bog Phase. The stormy period round about 

 1400 A.D. is well known, and has been attributed 

 by other authors to an absolute maximum of sun- ; 

 spots. The corresponding minima were in 2800 i 

 B.C., 1200 B.C., and 530 a.d., but of the 

 first of these there is no trace. The 

 second falls in the beginning of the 

 Peat-bog Phase, but there is evidence of sea- 

 borne traffic between Scandinavia and Ireland 

 about 1000 B.C., suggesting an absence of stormi- 



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