TRANSACTIONS OP THE SECTIONS. 47 



individual age is less than a century, and whose histories and traditions, however 

 freely rendered, fall short of a hundred centuries. The whole human period, as we 

 have been accustomed to view it, is but a unit in the vast sum of elapsed time : 

 yet in all those innumerable ages the same forces were seated ia the same particles 

 of matter; the same laws of combination prevailed in inorganic and in living 

 bodies ; the same general influences resided on the surfaces or governed the masses 

 of the planets, in their ever-changing paths rovmd the sun. 



All natural effects are perfonned ia time, and when the agency is imiform, are 

 in proportion to the time. And though the agency be not imifoiin, if the law of 

 its variation be known, the time consumed in producing a given effect can be 

 determined by calculation. Geological phenomena of every order can be expressed 

 in terms of magnitude, as the uplifting of mountains, the deposition of strata, the 

 numerical changes of the forms of life. The time required to produce these effects 

 can be calculated if we know at what rate in time, whether uniform or not, they 

 were produced : if we know, not the true rate, but the limits ■nnthin which it must 

 have operated, the result of the calculation will have a corresponding imcertaiaty ; 

 if we have no knowledge of the rate, calculations are out of the question. 



In applj-ing this general view to the history of the earth, philosophers of emi- 

 nence m physical science have employed diflerent considerations and obtained a 

 variety of results. The conclusions of two eminent mathematicians which have 

 lately appeared may be cited with advantage. 



A careful computation by Professor W. Thomson, on selected data, which deter- 

 mine the rate of^ cooling of earthy masses, assigTis 98,000,000 years for the whole 

 Seriod of the cooling of the earth's crust from a state of fusion to its present con- 

 ition ; so that, in his judgment, within one liundred millions of years aU our 

 speculations regarding the solid earth must be limited*. 



On the other hand, Professor Haughton finds, from the data which he adopts, 

 1018 millions of years to have elapsed while the earth was cooled from 212 F. 

 to 122° F., at which temperatiu-e we may suppose the waters to have become 

 habitable ; and 1280 millions of years more, ia cooling from 122° to 77°, which is 

 assumed to represent the climate of the later Eocene period in Britain. Com- 

 putations of this kind cannot be applied except on the large scale here exemplified ; 

 and they lose all their value in the eyes of those who deny the general doctrine 

 of a cooling globe t. Much as these periods exceed our conception, they appear to 

 be in harmony with the results of astronomical research, which contemplates spaces, 

 motions, and cycles of periods too vast for words to express, or numerals to coimt, or 

 symbols to represent. 



The greatest difficulty in obtaining trustworthy results as to elapsed time is 

 found where it was least expected — among the later cfenozoic deposits from rivers 

 and lakes, and on the variable shores of the sea. This is the more disappointing 

 because within this period falls the history of the human race. Taking as its earlier 

 limit the latest wide prevalence of glaciers in Europe, attempts have been made to 

 measure its diu'ation by several processes. Quite recently Mr. CrollJ recalls 

 attention to an astronomical cause of change of temperature — the varying excentri- 

 city of the earth's orbit — by which in a small degree the total qxiantity of heat 

 received in the earth in a year, and in a much greater degree the distrihdion of 

 this heat on the opposite cu-cumpolar spaces, are altered §. The effect of this at 

 particidar epochs would be, on one hemisphere an approximate equality of summer 

 and winter heat, on the other an augmented difference between them. If at the 

 epoch of maximum excentricity the earth was in aphelion dm-iug our winter, a 

 great accession of snow might arise and be continued for ages, and glaciers have a 

 large augmentation ; under the conti-ary circumstances, less snow and shortened 

 glaciers. To this latter condition the present state of the north corresponds ; and 

 by consulting the astronomical tables, it appears that a condition of extreme glaci- 

 ation, dependent on the maximum excentricity of the earth's orbit, cannot have 



* rhU. Mag. Jan. 1863. 



t Appendix to a Lecture on Geology, in the ' Eeader,' Feb. 1864. 

 X Phn. Mag. Aug. 1864. 



§ Consult on tliis subject generally the valuable communication of Sir J. Herschel to 

 the Geological Society, Proc. vol. i. p. 244, for Dec. 1830. 



