CHAP. X MEASUREMENT OF GEOLOGICAL TIME 233 



but that of the Swiss Lake dwellings, the Danish shell- 

 mounds, our peat-bogs, our sunken forests, and many of our 

 suj^erficial alluvial deposits — the whole in fact, of the iron, 

 bronze, and neolithic ages. Even some j^ortion of the 

 jmlseolithic age, and of the more recent gravels and cave- 

 earths may come into the same general period if they 

 were formed when the glacial epoch was passing away. 

 Now throuo-hout all these aoes we find no indication of 

 change of species, and but little, comparatively, of migra- 

 tion. We thus get an erroneous idea of the idcrmancnce 

 and stahility of siKcific forms, due to the period immediately 

 antecedent to our own being a iJCQ^iod of exccptioncd fcr- 

 manence and stahility as regards climatic and geographical 

 conditions.^ 



Date of Last Glacial Epoch and its Becoming on the 

 Measurement of Geological Time. — Directly we go back 

 from this stable j)eriod we come upon changes both in the 

 forms and in the cristribution of species ; and when we 

 pass beyond the last glacial epoch into the Pliocene period 

 we find ourselves in a comparatively new world, surrounded 

 by a considerable number of species altogether different 

 from any which now exist, together with many others 

 which, though still living, now inhabit distant regions. 

 It seems not improbable that what is termed the Pliocene 

 period, was really the coming on of the glacial epoch, and 

 this is the opinion of Professor Jules Marcou.^ According to 

 our views, a considerable amount of geographical change 

 must have occurred at the change from the Miocene to 

 the Pliocene, favouring the refrigeration of the northern 

 hemisphere, and leading, in the way already pointed out, 

 to the glacial epoch whenever a high degree of excentricity 



^ Tills view was, I believe, iirst put forth by myself in ca paper read 

 before the Geological Section of the British Association in 1869, and 

 subsequently in an article in Nature, Vol. L p. 454. It was also stated 

 by :Mr. S. B. K. Skertchlev in his Physical System of the Universe, p. 363 

 (1878) ; but we both founded it on what I now consider the erroneous 

 doctrine that actual glacial epochs recurred each 10,500 years during 

 periods of high excentricity. 



2 Explication d'une scconde edition de la Carle Geoloyiqnc dc la Terre 

 (1875), p. 64. 



