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OTOLOGY 



GEOLOGV. 



lead relative weight ; the earth must therefore b supposed to have 

 passed through a long range of condensation ; and thi implies a con- 

 tinual change of intensity among K>me at least of the physical 

 agencies which belong to it Whatever was the antagonist force to 

 toe central attraction of the nebular maw, the gradual decline of this 

 force mut have been felt, more or lea, by all the natural agencies 

 related to it by opposition or sympathy. Even the extraneous 

 influence of light is not independent of 'the change of conditions 

 produced. 



The continual condeniintion of the mam of a planet necessarily 

 brings with it a change in the relative intensities of the agencies at 

 work among iU part*, because they operate under continually varying 

 conditions. Some would lose and others gain in strength, and thus 

 the aspect of the earth must have been continually changing, or 

 subject to periodical renovation. By those geologists who accept the 

 doctrine of the earth's continual condensation, from whatever cause, 

 the uniform intensity of natural agencies taken separately, the con- 

 tinual compensation of their antagonistic effects, and the production 

 of equal effects in equal times, must inevitably be rejected. 



V. t though, in strictness, the preceding reasoning forbids assent to 

 Sir Charles Lyell's general principle, that the former changes of the 

 earth's surface " are referrible to causes now in operation," it by no 

 means follows that other causes (that is, other combinations or 

 measures of natural agencies) than those now in operation must be 

 appealed to for explaining the monuments of past revolutions of 

 nature which are preserved to our days. For if these monuments go 

 but a short way back on the scale of time, compared with the periods 

 which elapsed m the condensation of our planet, the causes may not 

 have sensibly varied during the whole course of phenomena traceable 

 in the crust of the earth. This must be decided by a study of the 

 monuments themselves, upon the general and acknowledged principle 

 that effect* are pro[K>rtional to the causes. Still less is it to be 

 imagined that the study of the effects of modern causes in action is 

 unfruitful in illustrations of the phenomena due to ancient causes; 

 on the contrary, there is no other way of learning either the kind or 

 degree of physical agencies concerned in geological operations of early 

 date than the comparison of these with the result* of the daily action 

 of the modern powers of nature. 



The knowledge of the condition of the earth with respect to tempe- 

 rature is one of the most important steps which can be taken toward 

 a right general contemplation of the history of the revolutions which 

 it has undergone. This knowledge cannot be gathered by geologists 

 labouring as such ; it cannot be obtained by meteorological observa- 

 tions, however accurate ; nothing short of a mathematical theory of 

 heat, supported by a variety of data concerning the physical consti- 

 tute >n and relations of the earth to the sun and space, will be at all 

 available in grappling with the inherent difficulties of the subject 

 For thU theory we are indebted to Fourier. 



The heat of any point on the surface of the earth regularly varies, 

 from hour to hour, with the rotation of the globular mass on its axis ; 

 from day to day and from season to season, with its revolution round 

 the sun ; and from year to year, with any change in the dimensions 

 or form of the earth's orbit There are however several causes of 

 irregularity or fluctuation of temperature not demanding notice in a 

 general view. 



If in its long course round the sun the earth passed through parts 

 of the planetary spaces of unequal temperature, this would cause a 

 modification of the periodical, annual, and daily variations. 



The atmosphere and the ocean by their various movements modify 

 all these circumstances, but not so as to disguise the results when an 

 average of many periods is taken. 



In consequence there is for each point of the earth's surface a cer- 

 tain mean temperature, depending on the causes above stated ; and 

 the parts under the surface continually tend to acquire very nearly 

 the same temperature as the surface, but not at the same time. The 

 extremes of summer heat and winter cold are not felt till after they 

 have pawed away from the surface; and in proportion as \ve descend, 

 the influence of the daily, monthly, and annual variations grows less 

 and leas, because of the slowness of the conduction of heat through 

 earthy substances. 



At a certain depth below the surface these variations become wholly 

 insensible, and the temperature is constant, and nearly the same as the 

 m-an temperature of the surface. 



If the temperature of the interior parts of the earth be now very 

 different from that constant heat which would result by communi- 

 cation from the surface (heated as before, and subject to the stated 

 variations), this difference would exercise a corresponding though 

 insensible effect on the surface heat, and be more or less sensible at 

 small depths below the inner surface of constant temperature. 



Whatever may have been the proper or original temperature of the 

 inner parts of the earth, it is easy to conceive that in a very long time 

 the equilibrium of heat should be reached, and the earth receive from 

 i anil radiate into th ethereal space equal quantities of heat 

 in equal times ; while the temperatures at point* situated at very 

 great depths below the surface (many miles, for instance) would not 

 enribly vary from that of the mean beat of the place vertically 

 above them. 



But if this equilibrium be not attained, the original state of the 



earth as to heat may be ascertained, so far as to determine positively 

 whether it has formerly been hotter or colder than at present, by 

 merely trying at many points exempt from volcanic action, what is 

 the amount of heat at various depths, on the same or different vertical 

 lines, as compared with the corresponding points of surface. 



These trial* have been made at various depth*, under different 

 circumstances, in salt-pits, coal-works, and mines of different metals, 

 in the British Isles, France, Germany, Mexico ; and in all situations 

 where the external influence of the air and the artificial effects of 

 light, respiration, Ac., could be guarded against or justly appreciated, 

 they agree in proving that after descending below the limit of variable 

 heat, a continual augmentation of temperature constantly occurs. 

 (1 Fahrenheit for 15 yards is a common ratio.) The mine of Kuhlun, 

 supposed to be an exception to this general truth, is extremely ill- 

 suited for experiment* (See Thomson's and Clarke's ' Travels in 

 Sweden.') 



The consequence is obvious. The interior masses of the globe are 

 incomparably hotter than the parts at the surface ; must form- rly 

 have been still hotter; and though now the interior heat is itlnit 

 wholly masked and stifled by the non-conducting stratified masses 

 which form the crust of *he earth, it nri-t f .rm.rly have influenced 

 in a decided manner the temperature, and with it all other phenomena 

 at the surface of the earth. 



The same conclusion as to the existence of great heat in the central 

 parts of the earth has been drawn from considerations of the d 

 of the interior masses as compared to the superficial parts. While 

 the surface rocks are twice and a half as heavy as water, the mean 

 density of the whole globe is five times as great as that of water ; 

 moreover the density augments towards the centre with so much of 

 regularity, that the imaginary interior surfaces of equal density are 

 symmetrical to the same centre and axis as those of the exterior 

 spheroid. (Conybeare's ' Report on Geology to British Association,' 

 1832.) Now, if the interior masses of the earth are compressible 

 even to a far less extent than the rocks near the earth's surface, the 

 pressure to the centre would have made the inner parts much more 

 dense than they are : the whole mass of the earth would have been 

 included in a much smaller volume were it not for some antagonistic 

 force, such as heat is known to be. Unless therefore we venture to 

 suppose the central and surface matter not subject to similar laws of 

 force, it must be admitted that the interior parts of the earth are still 

 very hot 



This great truth established, we may inquire further into the state 

 of the interior masses. If the heat of the globe were increased its 

 diameter would be augmented ; there is a degree of heat which would 

 liquefy nearly all the substances of which it consists, taken singly, 

 and still more easily when in their usual combinations. Beyond this 

 degree of heat gaseous compounds would mix with or altogether 

 replace the liquid rocks, and the globe would be lost in a nebulous 

 expansion. 



Turning to observations of phenomena, we find the interior rocks 

 to be such as were cooled from igneous fusion : they are exten- 

 perhaps universally, spread below our feet ; and thus we gather the 

 conviction that originally the whole or great part of the exterior 

 masses of the planet were in a melted state. The figure of the earth 

 is such as would result from revolution on its axis, provid> 

 whole or a very large part of the mass were in a state of fluidity or 

 viscidity ; to this figure the surfaces of equal density correspond both 

 as to centra and axis ; and thus strongly corroborate the specula- 

 tions of Leibnitz, that the earth is to bo looked on as a heated and 

 fluid globe, cooled and still cooling at the surface by radiation of its 

 superabundant heat into space. 



To determine whether it is now solid or partially fluid within is a 

 problem of high interest, and one which we may perhaps despair to 

 see completely solved, unlera certain HtrODOmioal phenomena (i>iv- 

 cession, nutation) should be found, when anahsed by a rigorous 

 mathematical deduction, to furnish interpretations which geology 

 alone can never attain to. As however Mr. Hopkins has presented 

 some simple views of the possible conditions of a cooling globe (as 

 the earth may be considered), we shall here briefly state them. 



If the earth wore originally a hot fluid mass cooled by radiation, 

 the cooled ports would descend towards the centre, and be replaced 

 by others in a perpetual circulation. The tendency to solidification 

 in such a mass would be directly as the pressure, inversely as the 

 temperature, both which are at a maximum at the centre : solidifica- 

 tion would therefore be determined near the centre by the super 

 of pressure over temperature ; and at the surfa< r by tin- rapidity of 

 external refrigeration overbalancing the internal conduction of heat. 

 The numerical relations of these qualities are unknown. It cannot 

 therefore be decided by mere calculation whether the solidification of 

 the surface by radiation would precede or follow that of the centre 

 by pressure. Let us suppose, for simplicity, the relations of pressure, 

 heat, circulation, conduction, and radiation to be such that all the 

 mass goes on cooling till every part of ita fluidity is lost, and the 

 whole is reduced to such a degree of vi-ei<!ity as to prevent the 

 circulation of heated matter, the further distribution of heat 

 under thcnc conditions, be determined by conduction and radiation 

 only ; a large part of the interior would assume equality of tempe- 

 rature: the solidification of the surface by cooling would be the 



