300 



DURATION OF THE GLACIAL, TERTIARY, [Ch. XIII. 



a maximum excentricity would only tend to render the 

 climate less equable, and not colder. If the ocean prevailed 

 in the polar regions there would be no snow, or no more than 

 the summer's thaw would dissipate ; and the difference in the 

 total quantity of heat being as 1003 to 1000, may, as Sir J. 

 Herschel observes, be neglected, and would, as before stated, 

 if appreciable, have a heating and not a refrigerating influence. 

 The attempt, therefore, to assign a chronological value to any 

 of our geological periods except the latest, must, in the pre- 

 sent state of science, be hopeless. We may imagine an ex- 

 treme excentricity and winter in aphelion to have sometimes 

 co-operated, with favourable geographical conditions, to 

 produce an excess of snow, and in this manner we may en- 

 deavour to account for some instances of local glaciation such 

 as those alluded to at pages 207 and 209, where the move- 

 ments of ice-action are intercalated in the middle of a series 

 of deposits, the fossils of which indicate a warm climate. 



Comparative duration of the Glacial and the antecedent Ter- 

 tiary, Secondary, and Primary Epochs. — But we obtain no 

 positive dates from such a source, and the utmost to which 

 we can aspire at present is to form some conjectures respect- 

 ing the relations of the Glacial Period to the present time, by 

 aid of such data as are afforded by the table given at p. 293. 

 Suppose, for example, in accordance with the views before set 

 forth, we could assume that the coming on of the Glacial 

 Period happened a million years before our time, we should 

 then have obtained some insight into the amount of change 

 in the marine testacea which that number of years has 

 brought about. Accordingly, when we find that ninety-five 

 in a hundred of the shells of the period alluded to were 

 specifically identical with those now inhabiting the northern 

 hemisphere, we may consider a million years to represent the 

 twentieth part of a complete revolution in species, and we 

 might thus estimate the number of years required for the 

 elaboration of the successive Tertiary formations ; nor should 

 we be in danger, according to the theory of developement or 

 transmutation of species, of exaggerating the rapidity of the 

 rate at which the old forms were supplanted by new ones, 

 because a general refrigeration of climate and several oscilla- 



