GEOLOGY. 



GEOLOGY. 



heap it on the surface of the earth. But the cavities left by thU 

 operation below the crust of the earth must often caune depression 01 

 I of land during the concussion and displacement* occasions 



by earthquakes. In this manner it may easily be understood that 

 the volcanic inlands of the South Seu have been raised up from the 

 aea-bed there, and it may be supposed that under large tracts of the 

 ocean volcanic agency is employed in a similar way, and by a 

 superiority of elevation over depression raising irregularly the bed o! 

 the sea, and by consequence extending the area of its surface. If all 

 the cavities left below the surface by the heaping of volcanic matter 

 on the land were completely balanced by corresponding depressions 

 of the crust of the earth it would depend upon the proportion of 

 submarine subsidence corresponding to terrestrial elevation whether 

 the sea-level should fall and its area contract Every sinking of the 

 sea-bad corresponding to an elevation of the dry land would tend to 

 lower the level of water and to augment the area of land. Along 

 sea-coasts such correspondence must be admitted occasionally to occur. 

 If the cavities alluded to were not compensated by the sinking of the 

 superincumbent crust volcanic phenomena on the land would hardly 

 affect the area or level of the sea ; but similar eruptions in the sea 

 would raise its level and cause it to encroach upon the land. If it be 

 admitted as the most probable basis of reasoning whether subter- 

 ranean cavities exist or not, that the continual elevation is upon the 

 whole balanced by continual subsidence, submarine and continental 

 volcanic vents may be left out of consideration ; but the littoral and 

 insular volcanoes act in one certain way, and give as the general 

 result of all volcanic action a partial deepening and a general con- 

 traction of the sea, which counterbalances in kind the general effect 

 of the aqueous agencies ; but whether these completely antagonist 

 principles are equal in degree cannot be safely inferred from any data 

 now accessible to geology. Nor does it appear prudent to rest so 

 important a conclusion on the mere fact of the constancy of the 

 earth's dimensions, indicated by the invariable length of the solar 

 day ; the experience of 2000 years is as nothing in a question of such 

 infinitesimal differences of diameter as might be occasioned by changes 

 in the relative position of the really small quantities of matter raised 

 or sunk by volcanic powers. 



Moreover it is impossible to avoid doubting whether even the 

 quantity of water on the globe is constant ; for so many combinations 

 of earthy substances require certain proportions of water for their 

 completion, and so much of volcanic excitement appears due to the 

 decomposition of water, that it would perhaps be safer to suppose 

 the water continually diminishing in quantity; nor is it at all unlikely 

 tbat such may be the case with the atmosphere. 



The question of the comparison of the effects of natural agencies 

 in modem and ancient times is one of considerable importance in 

 relation to geological enquiries. 



The statement of the effects of modern causes must necessarily be 

 received as true and applicable to other eras of the world, at least 

 in its general features ; because the chemical, mechanical, and vital 

 forces of nature are admitted as individually constant, though their 

 manifestations to our senses be ever so various in kind or degree, in 

 consequence of change in their combinations, the quantities of matter 

 operated on, external influences, &c. Fixed laws and variable 

 conditions are certainly recognised in existing nature, and they give 

 rise to extreme inequality in local results and combinations. It is 

 conceivable, by extending this idea, that the existing laws of nature 

 nhuuld be productive not only of results which, taken locally or 

 periodically, appear unequal in degree or diverse in kind, but that 

 under the influence of a general change of conditions they should 

 manifest a gradual decay or increase of strength, or spring into extra- 

 ordinary activity after long periods of apparent slumber. Let, for 

 instance, the sun's rays be supposed to fall upon the earth in smaller 

 quantity through the augmentation of the minor axis of the earth's 

 elliptic orbit ; let the temperature of the ethereal spaces rise : who 

 does not see that all the effect* depending on the external excitant 

 forces would immediately changer In like manner let the earth's 

 internal energy of heat be supposed to die away, whether for lack of 

 fuel, incrustation over metalloids, or a loss of general warmth in the 

 globe, the volcanic phenomena would be weakened, and no longer 

 balance the effect* of water. 



Now, as these great conditions cannot be affirmed to be constant, 

 but, on the contrary, as one at least of them is known to be variable 

 (the earth's orbit), how " baseless as the fabric of a vision " is the 

 assumption that the physical agencies on the globe have always 

 produced "equal effects in equal times," and that modern causes 

 acting with their present intensity have produced all the oldt r ].!,.- 

 nomena of geology. But it would be equally unjust, as observed 

 before, to assume that they hare not ; the question, if capable of 

 determination, can only be settled by ample observation and logical 

 induction. 



Among the ancient phenomena of nature we equally recognise the 

 contrasted action of water and heat, as at this day : by the former 

 the solid land was wasted, and stratified rocks were deposited along 

 the sea shores (as sandstones) and in the depths of the sea (as some 

 limestones), while the Utter manifested itself in the production of 

 uustratitied crystalline rocks, and the elevation and disruption of the 

 stratified bed of the sea. [ROCKS; STRATIFICATION.] The materials 



arranged by the action of water in the stratified rocks of ancient date 

 are the same as those now carried by rains, suspended by the tide, or 

 separated from sea-water by the vital functions of invertebrate ; they 

 are, to a certain extent, similarly associated : the organic < 

 buried in them are not very differently arranged or groupe.: 

 those which now lie in the bed of the sea (Donati's Researches 

 Bed of the Adriatic ' may be quoted in proof of this) ; the physical 

 conditions of their accumulation were therefore in a considerable 

 degree similar. 



On a careful consideration of the facts, it appear* obvious that the 

 long series of stratified deposits was not accumulated without great 

 and even sudden changes of those physical conditions : th. 

 of sandstone are followed by others of clay or of limestone, for 

 different agencies and conditions were required. Over the same 

 spherical area of the earth's surface the predominant physical con- 

 ditions varied from time to time, and many times, so that the actual 

 state of the globe, as far as regards watery agencies, represents not all 

 its previous conditions, but is to be compared with each of tin-: 

 ceasively. The same is true of the igneous products in the crust of 

 the globe, which similarly varied from time to time in the same 

 spherical area. 



Successive phases of the aqueous and igneous agencies over the 

 same region appear, either contemporaneously or successively, to have 

 affected all parts of the earth's surface accessible to man ; so that 

 everywhere there is proof of great revolutions in the condition of 

 land and sea. Moreover it appears [OROAMU REMAINS] that to each 

 general system of stratified rocks, indicative of a corresponding great 

 system of physical agencies, peculiar races of plants and animals 

 belong : with new physical conditions new forms of life came on 

 the globe, vanished with those conditions, and gave place to others 

 equally transitory. If now we compare the inoduru survey of nature 

 with any similar work, executed on the same principle, for any one of 

 the earlier epochs, it is certain that the earth h.ts iiinlergouo MMIIV 

 very extensive revolutions in all that respects its aqueous, igneous, 

 and organic phenomena, before arriving at its present state : it n 

 equally certain that between the epochs of these revolutions the state 

 of the earth was not extremely dissimilar to that which we now behold ; 

 yet, because the organic beings preserved in the earth in each of these 

 systems are peculiar to it, and differ from the others, and from those 

 that now live, we cannot possibly doubt that the points of difference 

 were numerous, general, and important 



To determine the cause of the change of physical conditions br 

 one system of stratified rocks and another is not difficult In existing 

 nature such a change might be easily produced in almost every region 

 by a disturbance of the level of some particular tracts of i.m.l, )>y 

 one great movement or many successive displacements. For ex > 

 let the Isthmus of Suez or the Isthmus of Darieii sink one huinireil 

 or a few hundred feet (perhaps scarcely beyond the range of the | 

 of an earthquake), what mighty changes would be occasioned in the 

 Indian, Mediterranean, Atlantic, and 1'aciac Oceans, over areas ln. h 

 would appear considerable even when compared with many ancient 

 systems of strata changes of stratified deposits and physical condi- 

 tions, and consequent variations in the relative abun<lainv ami geo- 

 graphical distribution of organic beings. Now, though at this day 

 no such mighty changes are witnessed, we have only to eiiluv 

 conception of the actual effects of volcanic agency to see clearly that 

 this is the power which was employed in producing th m. 



The analogy of the effects of aqueous and igneous agencies in all 

 past periods of the earth's history being assumed, we may prcxv 

 gather inferences as to the measure of the intensity \\itli which they 

 have operated, and the time which has elapsed during their operation. 

 This requires at least a brief summary of the chara itures 



of the phenomena of successive steps of the earth's formation, in t un 

 order of their occurrence. Observation can only guide us to a 1, 

 ledge of the crust of the earth for a depth of a few miles at i 

 and from what we there behold it is probable that a much greater 

 extension of the power of observing would really help us but little 

 in tracing the history of the revolutions of our globe "I which m ma 

 menu remain for inspection. For at some moderate depth ! ] th 

 surface all marks of lamellar increase, indicative of prno.iieal forma- 

 tion, cease; all monuments of life and watery action terminate; ami 

 we behold the effects of heat alone. The general basis of all the crust 

 of the earth, in which we trace the combined results a 

 aqueous, and vital energies, is a mass of crystallised rocks, the 

 Fruit of great aud very general heat; which limits all inquiry in 

 that direction. 



From the surface of these interior crystalline rocks, mostly of the 

 nature of granite, the monuments of physical changes left in the rocks 

 are capable of interpretation by the application of the knowled. 

 aave gathered of chemical, mechanical, and vital forces, but below it 

 all appears at first sight dubious and dark. Were these rocks of igneous 

 origin anterior to the whole crust of the earth now placed upon i i 

 Or does the interior heat slowly reconvert to granite t he masses of 

 sedimentary strata laid upon it by external watery agencies ! In the 

 ormer case the monuments of nature are complete to far a* any 

 hing analogous to the present system of surface agencies is 

 >ut according to the latter supposition, the earlier strata, with what- 

 ever of organic exuviiu lay in them, have been reabsorbed and melted 



