REFERENCE TO THE ANTIQUITY OF MAN. 



397 



not only here but on Bear Island and other northern lands. With 

 respect to the blocks of granite alluded to as occupying the lower beds 

 of the Coal-measures in France, they may be, like the Tors of 

 Cornwall, blocks left in situ from the decomposition of the granite 

 on which the Coal-measures there rest, or they may be boulders 

 washed down at that period by the torrents from the adjacent 

 granitic mountains. Other foreign pebbles may be accounted for 

 as we have accounted for those in the Chalk. 



There are similar pakeontological objections to ice-action in the 

 Devonian and Silurian periods. Although there may be at times 

 instances in which the blocks show striae and are derived from rocks 

 not known in the locality, it must be borne in mind that such 

 striated masses may be fragments of slickenside surfaces in the 

 rocks from which the breccias are derived ; and that, although a 

 particular rock may no longer show in the locality, it may exist 

 there buried beneath newer deposits, as, amongst others, in the case 

 of the granite of the Ardennes, which, although formerly unknown 

 there, was met with in a railway-cutting beneath a slight covering 

 of Palaeozoic rocks. 



Admitting the imperfection of the geological record, it is evident 

 that, as a whole, the physical instances fail entirely to supply any 

 sufficient corroborative evidence, either in force or in number, to 

 support the theory of recurrent Glacial periods. Surely out of 165 

 or even 100 cases, more definite marks would have been left, espe- 

 cially in the more recent periods, such as the Pliocene and Miocene, 

 when the land had assumed many of its present contours. 



With respect to the second point, Dr. Croll states that " the 

 Glacial epoch may be considered as contemporaneous in both hemi- 

 spheres. Put the epoch consisted of a succession of cold and warm 

 periods, the cold periods of one hemisphere coinciding with the 

 warm periods of the other, and vice verm''"' (p. 234). This would 

 involve an indefinite succession of interglacial periods ; but only 

 one definite interglacial period during the Glacial epoch is brought 

 forward. Dr. Croll, however, accounts for this on the grounds that 

 " the geological evidences of the cold periods remain in a remarkably 

 perfect state, whilst the evidences of the warm periods have to a 

 great extent disappeared (p. 238). If, however, one instance 

 could be well preserved, might we not expect other instances to 

 occur in some of the many localities affected ? Dr. Croll estimates 

 the average duration of a warm period at about 10,000 years. 

 Supposing such to have been the case, the phenomena of the glacial 

 series certainly afford no corroboration of the remarkable vicissi- 

 tudes of climate this would infer. 



There are, it is true, indications both in Pritain and Switzerland 

 of intervals of milder conditions during certain times of the Glacial 

 epoch ; but- these minor differences are, I think, due, not to the 

 cosmical cause of Dr. Croll, but to those changes of climate that may 

 be brought about by differences in the distribution of land and water, 

 such, for example, as the extensive submergence which took place in 

 England and North Germany after the first great land-glaciation, 



Q.J.G.S. No. 171. 2e 



