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Show 140 CllANGES OF TilE SURFACE the equator and the tropic in one hemisphere to be to that in the other as thirteen to one, which, as we before stated, represents the unequal proportion of the extra-tropical lands in ~he two hemispheres at present. Then let the first geographical change consist in the shifting of this preponderance of la?d from one side of the line to the other from the southern hemisphere, for example, to the northern. 'Now this would not afl'ect the general temperature of the earth. But if, at another epoch, we suppose a continuance of the same agency to transfer an equal volume of land from the torrid zone to the temperate and arctic reO"ions of the northern hemisphere, there might be so great a ref~igeration of the mean temperature in all latitudes, that scarcely any of the pre-existing races of animals would survive, and, unless it pleased the Author of Nature that the planet should be uninhabited, new species would be substituted in the room of the extinct. We ought not, therefore, to infer, that equal periods of time are always attended by an equal amount of change in organic life, since a great fluctuation in the mean temperature of the earth, the most influential cause which can be conceived in exterminating whole races of animals and plants, must, in different epochs, require unequal portions of time for its completion. The only geological monument yet discovered, which throws light on the period immediately succeeding the deposition of the chalk, is the series of calcareous beds in St. Peter's Mount at Maestricht. The turtles and gigantic reptiles there found, seem to indicate that the hot climate of the secondary era had not then been greatly modified ; but as it seems that but a small proportion of the fossil species hitherto discovered are identical with known chalk fossils, there may perhaps have been a considerable lapse of ages between the consolidation of onr upper chalk, and the completion of the Maestricht group*. During these ages, part of the gradual rise of the Alps anrl Pyrenees may have been accomplished; for we know that earth· * It appears from a Memoir by Dr. Fitton, read before the Geological Society of London, Dec. 1829, that the Maestricht beds extend over a considerable area, preserving the same mineral characters and organic remains. Out of fifty species of shells and zoophytes collected by him, ten only could be identified with the copious list of chalk fossils published by. Mr. Mantell, in the Geol. Trans., vol. iii. part 1., second series, p. 201. AND CLIMATE" ' CONTEMPORANEOUS. 141 quakes may work miO'ht I 11 sma portiO. n of one o z Y1 c 1a.n cres du · 1 o n~g w lat we may call a Sicily which have O'ain odo ogiCal era, smce there arc hills in h . . o c more than th. I e1ght, wiule the assembi f Iee t 1ousand feet in biting the Mediterraneana~e o \estacea and zoophytes inhaand a large part of the co ats. on by suff~red slight alterations h un nes ordent t1 1\I d. ' ave been remodelled sine b . Jg 1e .u e Iterranean . . e a out one-third of tl . . c1es were m being. 1e ex1stmg spe- Before we conclude this cha ter offer some remarks on the rad p 1 j- w~ may be expected to central heat of the globe g dua ~ Immu~ion of the supposed ' a octrme w1uch years to have increased in P 1 . appears of late k . opu arJty 13aron F . r·n a mg a curious series of experi. me·n ts t1 our1er., aftet· Incandescent bodies, has endeavoured b .on le cooimg of cal calculations to prove th t tl Y P1?found mathematiin the earth's envelope l·s a ~e }actual distribution. of heat · prec1se y th t h · h taken place if the O'lobe had b ./! ~ w IC would have h . h o een 10rmed m d' tg temperature and 1·1 d f a me lllm of a very ' c a a terwards b He supposes that the matter of 1 een constantly cooled ''If.. conjectured, was in an l'nte 1 oulr p anet, as Leibnitz formerly . nse y 1eated st t h ~reatwn, and that the incandescent fl 'd a e at t e era of its mg ever since with portions of 't u~ . nucleus has been part-t ractm· g I·t s d. I s origmal h t 1 1mensions _ . . ea , t 1ereby con-d ' a process wluch h cease . But it is admitted ' til·a t tl1 e re are ans not .y .e t entirely support of this contraction. on th o positive facts in s?ewn, by reference to astr;nomicat oc~ntmr~, La Place has time of Hipparchus, that in the last Bservatwns made in the ~as been no sensible contraction of th tw~ t~wusand years there or had this been the case e g o e by cooling down th e d ay would have been' sehvoenrt to adn . ext reme 1y small amount,' Th. e reader will bear· l·n mm. dene] m an appreciable decrree t lat th . o • existence of a central h t . ' . e q uestwn as to the d ea IS very d ffi . J'. gra ualrefricreration of tl . t . 1 erent Hom that of the ~ati· ons 0 1e m enor of th h and experiments a e eart . Many obser-m descending from th pyear to countenance the idea that e SU1'1ace to th J' h ' man can penetJ·ate ther . ose s Ig t depths to which 1·f t1 u ·s be establishe' d aned 1 s 'af proOo 're ssi·v e I.n crease of heat; but ' 1 ' as some are not afrai'd t om. fie r, * Sec a Memoir on tho Tem tary Spaces, Ann. de Chimic et ~~raturte of the.;rerrestrial Globe, and the Plan~ lys., om. xxvu. p. 136, Oct. 1824, |
