624 



SCIENCE. 



SHALER AND DAVIS' "GLACIERS." 

 By W. J. McGee. 



(Continued from page 584). 



VII. Ancient glacial periods. — Since the records of 

 glacial phenomena are mainly such as are likely to be ob- 

 literated by succeeding' geological mutations, it is need- 

 less to look for as unequivocal testimony of glacial pe- 

 riods as that attesting the occurrence of the last ice age. 

 The principal evidences we can hope to obtain are (i) 

 tide-washed boulders, gravel, and sand, (2) erratics 

 dropped from icebergs in the deeper sea deposits, and 

 (3) paucity or absence of organic remains, or possibly 

 fossil forms suggesting low temperature ; and it is to be 

 expected that such evidence will become constantly less 

 explicit as the geological record is traced backward. 



Anterior to the Quaternary ice period, we first come 

 upon evidence of a Miocene glacier, well marked in the 

 hill of Superga, near Turin ; trom which evidence it may 

 be inferred that the European continent underwent a 

 very severe glaciation in Miocene times. Next follows the 

 Eocene, in which the Flysch of Eastern Switzerland, 

 formed of conglomerates containing immense boulders, 

 supposed to be derived from worn-down mountains of 

 the Vosges or Black Forest, eloquently attests vigorous 

 ice-work; while "in North America we can almost 

 mark the line of the ice by the limit of the destruction 

 of the Tertiary beds " (p 95), and are hence w.thout so 

 characteristic ice deposits. The Cretaceous affords no 

 evidence of glacial conditions save occasional iceberg- 

 dropped boulders ; but in the Jurassic are found the con- 

 glomerates of Northern Scotland and of the Connecticut 

 Valley, which appear to be of glacial origin. The traces 

 of Permian glaciers are unmistakable, and of almost 

 world-wide extent, being found in Central England, in 

 Scotland, in the Isle of Arrcn, in Ireland, in South 

 Africa, and elsewhere. The rocks of Carboniferous age 

 include conglomerates, probably of glacial origin, from 

 Southern France to Scotland, from Alabama to New 

 Brunswick, in India, and in other countries ; from which 

 it appears " that it extended ice action over a wider me- 

 ridional range than any [periods] that have succeeded it " 

 (p. 98). There are then no beds certainly a'testing ice- 

 work until the base of the Cambrian is reached, when 

 the extensive Ocoee conglomerates and Chilhowee sand- 

 stones, and lesser deposits of like character at Roxbury, 

 Mass., and elsewhere, are found. In many of the fore- 

 going cases the associated beds indicate warm or sub- 

 tropical climate, as should happen according to Croll's 

 theory. 



VIII. The cli matal conditions of theg lacial periods. — 

 There is no reason to believe that ice periods were ever of 

 particularly low temperature. On the contrary, the Qua- 

 ternary fauna was characterized by the great size of its indi- 

 viduals, and clearly proves the contemporaneity of a luxu- 

 riant flora, such as could not exist in arctic cold. More- 

 over, there are abundant proofs, that this glacial period 

 was a time of much greater raintall than the present. Ac- 

 cordingly several glacial hypotheses may be summarily 

 dismissed. That of Foisson is qute untenable. It is 

 questionable whether that attributing climatal oscillation 

 to variation in the constitu ion of the atmosphere should 

 be admitted to have weight ; though effects resulting 

 from such a cause would t>e cumulative. Croll's theory 

 is found to harmonize strikingly with the observed facts ; 

 There is the last period of high eccentricity occurring at 

 the proper date for the Quaternary ice age ; there are 

 the numerous successive inter-glacial periods correspond- 

 ing with the alternate advances and retreats of the ice; 

 and there are the brief epochs of warm climate, repre- 

 sented by luxuriant floras and vigorous faunas. On the 

 other hand, however, there are the objections that some 

 minor factors may possibly have been overlooked in fram- 

 ing the theory ; that there are vast ages without evidence 

 of glacial action, as should not occur, according to the the- 



ory, since " the eccentricity of the earth's orbit is such a 

 constantly recurring phenomena" (p. 107) ; that the Ant- 

 arctic glaciers are not advancing, but that all observations 

 " lead us to the conclusion that the ice there is as much 

 in process of retreat as it is in the Northern Hemisphere " 

 (p.. 107); and that, the hypothesis assumes essentially 

 the same outline for Cape St. Roque during the glacial 

 periods as at present. But none of these objections are 

 fa'al to the theory ; the only question is as to its being 

 a sufficient cause of anything so wide-spread as the con- 

 tinental ice periods. There are also elements of proba- 

 bility in the hypothesis of Le Coq, that variations in solar 

 emissivity might produce glaciation ; for augmented tem- 

 perature would increase precipitation and lead " to an 

 extension of the fog envelope which in all glaciated regions 

 does so much to protect the ice from the sun " (p. 108 ); 

 but on the whole, though this hypothes s has the advant- 

 age of ndefiniteness, it is entitled to less weight than that 

 of Croll. Neither must the hypothesis of minor geogra- 

 phical alterations be overlooked, since it is perhaps possi- 

 ble that such changes may have occurred in such manner 

 as to facilitate glaciation. The question, however, re- 

 mains an open one. " and it is unsafe for the geologist to 

 commit himself definitely to any of 'he hypotheses that 

 have been suggested " (p. 110). There are half a dozen 

 distinct and powerful causes, together with a number of 

 minor factors, which co-operate to produce the singular 

 uniformity of terrestrial temperature ; and the only safe 

 conclusion is that the earth's secular winters may be due 

 partly or wholly to any or all these agencies. 



IX. Effect of glaciers on the altitude 0/ the lands. — 

 Throughout northern regions there are evidences of con- 

 siderable depression of the land during the glacial period ; 

 which depression in Europe was variable, and mamly con- 

 fined to the severely glaciated area. " In America this 

 depression has not been studied except along the Atlan- 

 tic shore " ( p. 113), where it increases from twenty feet 

 near the southern limit of glaciation, to over two thousand 

 feet in Greenland. The land appears to have remained 

 below the normal level until after the withdrawal of the 

 ice. Two hypotheses have been framed to account for 

 this depress on : — 1st. That of Adhemer, which attributes 

 the phenomenon to the dislocation of the earth's centre 

 of gravity by a potar ice-cap, and which is based on the 

 assumption of a rigid terrestrial crust. The depressions 

 at various lati'udes were not, however, of the relative 

 value demanded by this hypothesis. 2nd. That of local 

 deformation of a flexible terrestrial crust beneath the 

 weight of the ice. This h\pothesis is supported by the 

 greater part of the evidence thus far collected ; though it 

 is likely that both classes of agencies co-operated in pro- 

 ducing the effect. There is reason to believe, also, that 

 a temporary upheaval of lands sou h of the ice-sheet oc- 

 curred during the Quaternary; which upheaval was 

 doubtless a concom tant of the local depression beneath 

 the ice. 



X. The effect of glaciation on the life of the earth. - 

 During the growth of the ice-sheets there must have been 

 a widely extending southerly migration of animals and 

 plants, giving rise to individual and specific variation in 

 consequence, not only of the change of habitat, but 

 also of the crowding of individuals over the contracted 

 habi able area ; and the converse movements following 

 the withdrawal of the ice must have been nearly as im- 

 portant biologically. The development and extinction of 

 the hairy mammoth affords an illustration of the effects 

 of secular winter on animal life. " In the closing stages 

 of the glacial period we find him the most widely dissem- 

 inated of all the large mammals that are known to us " 

 (p. 1 19) ; his remains occurring alike over Europe, Asia, 

 and America. " As we go back into the glacial time, we 

 have fewer and fewer indications of the extstence of this 

 noble beast, yet we have remains enough to make out 

 that he or his immediate ancestors existed at the begin- 

 ning of that epoch, and that in all its stages he was feeding 



