396 



PROF. J. PBESTWICH OX THE GIACIAL PEBIOD, WITH 



Postglacial times, could not be less than 80,000, whilst, if 

 carried back to Pregiacial times, it would necessitate an 

 antiquity of 200,000 to 300,000 years. 



With regard to the first point, Dr. Croll shows that in the three 

 million years for which his tables are computed there were five 

 periods during which the eccentricity was as great as or greater than 

 during the Glacial epoch proper ; so that, taking geological time at 

 the hundred million years, at which he estimates it, there should 

 have been in all probability some 165 such periods of cold. With 

 the exception of the Permian, which is still "pendente lite," where 

 is there evidence of any such cold periods ? 



Dr. Croll does not overlook this difficulty, and contends that in 

 the Italian Alps there is strong evidence in favour of the opinion 

 that glacial conditions existed there during the Miocene period. 

 This, he informs us, is stated on the evidence of two distil] guished 

 geologists ; but the fact of the Miocene date of the beds in question 

 has never since been confirmed ; all the later evidence tends to 

 show that it was not until towards the close of the Pliocene period 

 that glacial conditions set in. At the same time he admits the 

 existence of warm conditions, as undoubtedly proved by the flora, 

 in Greenland during Miocene times. 



For the Eocene period Dr. Croll relies on another Alpine case — 

 the coarse conglomerates with some enormous blocks forming the 

 Flysch. But this was a period of Alpine disturbance and change, 

 when, though the rocks ma3 T have been rent and worn down in the 

 mountain area, the marine life at a short distance gives evident indi- 

 cations of a high general temperature : Nummulites then abounded 

 in the surrounding seas, together with Echinoderms of a decidedly 

 tropical aspect. 



The case for the Chalk is still weaker, for the very few and 

 exceptional foreign rock-boulders that have been found in it are 

 of small size, such as might have been carried in the roots of trees 

 or by seaweeds, or possibly by small winter ice-rafts from the 

 mountains of Scandinavia or the Ardennes, whilst all the life of the 

 Cretaceous sea is strictly that of temperate, if not of warm, latitudes. 

 The small pebbles may have been carried by the large marine reptiles. 



It is on facts of the same character as those which Dr. Croll 

 adduces for the Eocene period that he would found evidence of the 

 action of ice in Scotland during the Oolitic period ; but we must 

 seek for some other explanation to account for the dispersion of 

 the conglomerates d,nd boulders in face of the incompatible fact that 

 at those times warm conditions of climate extended to 70°-80° 

 jSTorth, and that corals, Cephalopods, and huge reptiles swarmed in 

 the seas. 



The climatal conditions during the Permian period may be open 

 to doubt ; but on this yet unsettled point it is not necessary here to 

 enter. If admitted, it would not affect the general question. 



]Nor is it easy to admit a claim for ice-action during Carboniferous 

 times when the luxuriant vegetation of the Coal-measures flourished 



