236 



PLEISTOCENE 



PLESIOSAURUS 



the weight of the great ice-sheeta which covered 

 such vast regions in our hemisphere during glacial 

 times may nave displaced the earth's centre of 

 gravity, and thus caused a rise of the sea in the 

 north. Others, again, think it probable that 

 under the pressure of a great ice-sheet the earth's 

 crust may have yielded and sunk down more or 

 less gradually. Some, again, have thought that 

 u thick ice-sheet would exercise sufficient attrac- 

 tion upon the sea to cause it to rise upon the land. 

 It must be admitted, however, that some of the 

 oscillations of level which took place in Pleistocene 

 times were on much too considerable a scale to be 

 explained by any of the hypotheses referred to. 

 If, for example, the considerable submergence 

 which happened just before the advent in the 

 British area of the last mer de glace was due 

 to the pressure of an ice-sheet covering some 

 region farther north, it is hard to understand how 

 great confluent glaciers afterwards succeeded in 

 covering an area lying several hundred feet below 

 the surface of the sea. Again, if the crust of the 

 earth were so readily deformed under the weight 

 of an ice-sheet, how shall we explain the absence 

 of any traces of contemporaneous marine action 

 in those low-lying regions of south England and 

 the Continent which were invaded by the great 

 northern ice-sheet? \Vliatever influence the ice- 

 sheets of the glacial period may have had upon the 

 sea-level, it seems most probable that the greater 

 oscillations were the result of considerable earth- 

 movements, such as have taken place at many 

 different stages of the world's history. 



Pleistocene in other Continents. In North Amer- 

 ica deposits of the same character and showing 

 the same general succession as those of Europe are 

 encountered the glacial and interglacial conditions 

 that characterised the latter continent having been 

 equally characteristic of the former. The Pleisto- 

 cene fauna of North America embraced Mastodon, 

 a true elephant, species of horse, bison, beaver, 

 peccary, bear, &c., and gigantic extinct forms of 

 sloth, such as Megatherium, Mylodon, and Mugal- 

 onyx. In South America the Painpa deposits 

 have yielded a large number of remains of the 

 great sloths and armadillos ( Glyptodon ), besides 

 other mammals. There is abundant evidence also 

 to show that snow -fields and glaciers had in 

 Quaternary times a considerable development in the 

 Cordilleras, while in Fuegia ice seems to have over- 

 flowed much of the low grounds. In South Africa 

 former snow-fields and glaciers have left their 

 traces in the Kaga and Krome mountains. So, 

 again, New Zealand and Australia seem to have 

 had a glacial period. Little is known of the moun- 

 tains of central Asia, but old moraines and erratics 

 have frequently l>een observed in the Chinese 

 ranges, while, as is well known, the glaciers of the 

 Himalayas had formerly a very great development. 



Cimse of the Pleistocene Climatic. Changes. Many 

 speculations as to the cause of the climatic changes 

 of Pleistocene times have been indulged in. It 

 must be admitted, however, that none of these 

 explanations is without ito difficulties. Hut the 

 theory which liest accounts for the fact and has 

 gained the widest acceptance is that advanced by 

 the late Dr ('roll. According to him, the strongly 

 contrasted climates of the Pleistocene period were 

 the indirect result of the increased eccentricity of 

 t In- earth's orbit, combined with the precession of 

 the equinox. During a period of high eccentricity 

 that hemisphere which had it winter in aphelion 

 would be subject to severe cold, and its snows 

 would not diap|>ear during the succeeding short 

 summer, the temperature of which would be 

 lowered by ite presence. Thus in time that hemi- 

 sphere would tie subjected to severe glacial condi- 

 tions. In the opposite hemisphere the state of 



tilings would be very different, for the winter 

 would be short and mild and snow would not 

 ;u cumulate. Under such widely contrasted con- 

 ditions between the northern and southern heini 

 spheres, the trade- winds, and through them the 

 great equatorial ocean-currents, would be power- 

 fully atlected. The winds blowing from the glaci- 

 ated lii'ini-phere would be much stronger than 

 those coming from the other, and the equatorial 

 oceanic currents would thus be impelled across the 

 equator into that hemisphere whose summer hap- 

 pened in aphelion. This influx of warm water 

 would necessarily increase the temperature of that 

 hemisphere, while the other would have its tem- 

 perature correspondingly lowered. Owing to the 

 precession of the equinox, however, the conditions 

 of the two hemispheres would tend to be reversed 

 every 11,000 years or so; so that during a cycle 

 of great eccentricity each hemisphere would ex- 

 perience an alternation of extremely cold and very 

 genial climatic conditions. The last |<e>iod of high 

 eccentricity commenced some 240,000 years ago 

 and lasted for about 160,000 years. Some geol- 

 ogists have objected that the glacial period cannot 

 be carried so far back in time. Thus, from an ex- 

 amination of the rate of erosion in the postglacial 

 gorges of some rivers in North America (Falls of 

 Niagara, Falls of St Anthony ), some observers con- 

 clude that only 7000 or 8000 years have elapsed 

 since the close of the ice age. But in all such 

 measurements and estimates there are elements of 

 uncertainty which render the conclusions based 

 upon them of little value. 



Antiquity of Man in Europe. All the human 

 relics met with in Pleistocene deposits lielong to 

 what is known as the Palaeolithic stage. Hitherto 

 no indubitable evidence is forthcoming to show 

 that man was an occupant of Europe Iwfore the 

 glacial period. It is quite possible lie may have 

 been, but we lack evidence sufficient to prove 

 this. He certainly lived, however, throughout the 

 glacial and interglacial conditions descril>e<l above. 

 It is remarkable that no trace of his occupation has 

 lieen met with in beds of later date than those 

 pertaining to the close of the last interglacial 

 epoch. If we were to judge from negative evi- 

 dence (which it is always unsafe to do) we should 

 infer that he vanished from Europe during the 

 last glacial epoch. The oldest human relics 

 hitherto discovered in postglacial beds are Neo- 

 lithic. 



See Crpll's Climate and Time (1875). Various objec- 

 tions which have been urged against this theory have 

 been answered by its author in his Climate and Cot- 

 molofiy (1886). See also G. F. Wright, The Ice Age in 

 North America (1889), and the articles in this work on 

 GEOLOGY, ANTHROPOLOGY, MAN, EUROPE, STONE AGE. 



Plenipotentiary. See AMBASSADOR. 



Plesiosniims (Gr. plesios, 'near to,' sanros, 

 'a lizard'), the type or leading genus of a family 

 ( Plesiosauridip ) of fossil sea-reptiles, which are 

 characteristic of the Mesozoic systems. The skull 

 of Plesiosaurus is small and depressed, with a short 

 mandibular symphysis. The teeth (sunk in dis- 

 tinct socket*) are long, slender, and cylindrical, 

 and show fine longitudinal ridges on the enamel. 

 The most striking peculiarity of the vertclir.-e is 

 the great length of the neck portion, which was 

 composed of from '24 to 41 vertebrae the anterior 

 ones being generally very small. The cervical 

 vertebra; consist of a centrum, neural arch, and 

 two ribs, which are firmly articulated to the centra 

 of the vertebnp, the terminal faces of which are 

 more or less biconcave. In the dorsal vertebras 

 the ribs are articulated to diapophyses from the 

 neural arch ; and in tin- tail they gradually descend 

 again to the sides of the centrum. The tail is 



