H6 DISCOURSE ON THE STUDY 



been recognised by geologists, from their examin- 

 ation of the remains of animals and vegetables of 

 former ages enclosed in the strata, various causes 

 for such diminution of temperature have been as- 

 signed. Some consider the whole globe as having 

 gradually cooled from absolute fusion ; some regard 

 the immensely superior activity of former volcanoes, 

 and consequent more copious communication of in- 

 ternal heat to the surface, in former ages, as the 

 cause. Neither of these can be regarded as real 

 causes in the sense here intended ; for we do not 

 know that the globe has so cooled from fusion, nor 

 are we sure that such supposed greater activity of 

 former than of present volcanoes really did exist. 

 A cause, possessing the essential requisites of a 

 vera causa, has, however, been brought forward* 

 in the varying influence of the distribution of land 

 * LyelPs Principles of Geology, vol. i. Fourrier, Mm. de 

 1'Acad. des Sciences, torn. vii. p. 592. " L'6tablissement et 

 le progres des societ^s humaines, Faction des forces naturelles, 

 peuvent changer notablement, et dans de vastes contrt5es, l'e"tat de 

 la surface du sol, la distribution des eaux, et les grands mouve- 

 mens de 1'air. De tels eflfets sont propres a faire varier, dans 

 le cours de plusieurs siecles, le de"gre de la chaleur moyenne ; 

 car les expressions analytiques comprennent des coefficiens qui 

 se rapportent a Ptat superficiel, et qui influent beaucoup sur 

 la valeur de la temperature." In this enumeration, by M. 

 Fourrier, of causes which may vary the general relation of tho 

 surface of extensive continents to heat, it is but justice to Mr. 

 Lyell to observe, that the gradual shifting of the places of the 

 continents themselves on the surface of the globe, by the abrad- 

 ing action of the sea on the one hand, and the elevating agency 

 of subterranean forces on the other, does not expressly occur 

 and cannot be fairly included in the general sense of the passage, 

 which confines itself to the consideration of such changes as 

 miy take place on the existing surface of the land. 



