993 



GEOLOGY. 



GEOLOGY 



991 



of past geological periods ? Three orders of effects are in this respect 

 important : 



1. The deposition of stratified rocks. 2. The changes of organic 

 life on the land and in the sea. 3. The displacements of land, and 

 changes of physical geography. 



The phenomena of stratification are at this day repeated, aud on 

 a very considerable scale, in most parts of the world. Where great 

 rivers sweep earthy materials and vegetable reliquiae to the sea, as 

 in the case of the Mississippi, Amazonas, Rhine, the Po, and other 

 rivers, littoral aggregations take place, and new land is formed ; tides 

 and currents throw up sand-banks, or disperse the finer sediment far 

 from the shore over the quiet bed of the ocean. From the growth of 

 new land on the Adriatic and Egyptian coasts, by the action of the 

 Po and the Nile, some notion may be formed of the great quantity 

 of sediment annually transported by rivers to the sea, aud both 

 reason and experience show that the materials are there accumu- 

 lated in the same manner as the ancient strata were. 



But are they now accumulated with the same, with greater, or 

 less rapidity ? If equal deposits are now formed in equal times, the 

 calculation of the age of the visible crust of the earth is as easy as it 

 would be philosophically useless ; but to assume this principle is to 

 nullify the conclusion from it. Unless it can be shown, a priori, 

 that atmospheric influence must have been constant through all past 

 geological time, the assumption will not be accepted. This cannot 

 be satisfactorily shown, for the external excitants on which the 

 atmospheric actions depend contain variable elements. No certain 

 conclusion then can be rested on the comparison of the mere thickness 

 of the stratified rocks, as to the lapse of time, unless there can be fouud 

 an independent scale of time which may help to interpret the other. 



Such a scale of time is perhaps contained in the series of organic 

 beings imbedded in the earth. These belong to many successive 

 systems of life, which may be compared with the existing forms of 

 nature, and- could we establish from history any rate of change in 

 organic life, any per-centage of species destroyed, or created in a given 

 series of years, some considerable steps might be laid for further 

 advance. But two or three thousand years appear to have made no 

 change on quadrupeds, birds, reptiles, fishes, shells, or conspicuous 

 plants. As far as can be known by study of old writers on natural 

 history, sculptured monuments, coins, and mummies, no change of 

 external form or internal structure has been experienced since the 

 earliest historical era ; the loss of a very few species is all that can 

 be safely admitted ; and no proof is offered of a single newly-created 

 form, though the distribution of the different groups of plants and 

 animals has been varied by sea-currents carrying seeds and ova, 

 aud altered by man, who has learned to conquer by obeying nature. 



As far therefore as the more obvious and characteristic forms of 

 animals and plants can be admitted to yield satisfactory evidence, 

 the period of two thousand years since the days of Aristotle would 

 be insufficient even as a uuit of measure by which to estimate the 

 intervals of geological time which elapsed during the deposition of 

 strata. This conclusion is strengthened by some and weakened by 

 other considerations. It is weakened by tho circumstance that the 

 changes of organic life appear to have been sudden ; it is fortified 

 and illustrated in a powerful degree by comparing existing nature 

 with the tertiary era, for thus the ten or more thousand shells of 

 thig day appear to be joined to an equal number of others, into one 

 long series of definite organic forms, which, since the date of the 

 chalk, have admitted new and lost old species continually. Whether 

 these new" species, in any particular basin of strata, were parts of 

 one or more new creations there, or, as may perhaps be thought 

 probable, transferred from other centres of oceanic life, is quite 

 unimportant for the argument as to time. The effects resemble those 

 noticed among the older strata, the causes must be assumed to be 

 correspondingly similar, and the times must be in some degree 

 proportionate. Uniting therefore the tertiary and modern eras into 

 one great geological period, we may compare the unknown quantity 

 of time which it includes with other equally unknown and older 

 intervals in the history of the globe, correspouding to similarly 

 complete series of organic forms. This comparison is facilitated by 

 the remarkable fact of the almost total distinctness of the organic 

 beings of successive geological periods. Had the shells of successive 

 systems of strata been gradually changed by substitution, we should 

 hate been compelled to. compare not systems but formations, or 

 even individual strata; and the conclusions might have become 

 irremediably obscure. 



The systems to be compared are : Tertiary, Cretaceous, Oolitic, 

 Saliferous, Carboniferous, Fossiliferous, and Primary. 



The following table, extracted from Professor Phillips's ' Guide to 

 Geology,' gives the proportionate thickness and number of organic 

 forms of these systems : 



Number of species of organic 



Strata. General thickness. remains to 100 ft. thickness. 



Tertiary . . 2000 feet . . .141 

 Cretaceous . . 1100 feet . . . 707 

 Oolitic . . 2500 feet . . . 45'6 

 Haliferoua . . 2000 feet . . . 8 '2 

 Carboniferous . 10,000 feet ... 47 

 Primary . . 20,000 feut . . . 2'0 

 KiT. HIST. D1V. VOL. II. 



Hence it is very obvious that any conclusions as to time, drawn 

 from the mere number of species which were developed and destroyed 

 with any system of strata, will be totally opposed to others based 

 on the observed thickness of the strata. The inferences are obvious 

 aud important ; the numerical relations of organic life to the amount 

 of stratified deposits are variable; one cannot be used as a measure 

 of the other ; the variety and abundance of organic life has been 

 augmenting from the primary to the tertiary eras, or the deposition 

 of strata was in the early ages of the world fifty times as rapid as 

 in the tertiary period. This latter conclusion can never be allowed, 

 since the fossiliferous primaries show clearly their origin from laud- 

 floods and littoral currents, aud these depend on influences which 

 cannot be supposed to have varied in any such proportion. 



It thus appears that neither tho numbers of organic fossils nor the 

 thicknesses of strata afford a perfectly satisfactory scale by which to 

 measure past geological time ; but whichever of them be preferred, 

 the age of the world cannot be estimated at less than several times 

 the whole tertiary period, and compared with this the historical 

 portion of time, which dates from the birth of man, contracts to a 

 point. 



By uniting the two considerations above stated, it will appeal- 

 certain that the rate of organic development has been augmented, 

 and probable that the rapidity of sedimentary deposition diminished 

 since the primary era ; and it is no slight argument in favour of tho 

 hypothesis of a gradually cooling globe, that both these phenomena 

 are natural consequences of it for that the greater influence of the 

 earth's proper heat in the earlier epochs would favour the mechanical 

 but limit the vital activity of nature seems to require no proof. 



If however independent proof were required of this change of 

 ratio among the agencies of nature, we must appeal to a third order 

 of phenomena most certainly characteristic of disturbances of the 

 equilibrium of the earth's proper temperature : the fractures, con- 

 tortions, and other marks of the violent elevation aud depression of 

 the crust of the globe. 



From what has been already stated it is very clear that the 

 principal phenomena of this description occurred specially at particular 

 intervals during the long periods of geology ; for example, after the 

 primary period, after the carboniferous era, before and after the 

 accumulation of the cretaceous strata, after many of the tertiaries 

 were produced. Now, on comparing the amount of disturbance 

 effected at these epochs respectively, we are unable to perceive that 

 the efficient causes have diminished in force ; for the elevation of the 

 Alps in the tertiary period is apparently quite as conspicuous a 

 phenomenon as can be found among older geological monuments. 

 M. Elie de Beaumont, to whose speculation as to the geographical 

 characters of subterranean movements allusion has already been 

 made, supposes that as many as twelve distinct epochs of mountain- 

 elevation may be recognised. The following is a brief summary of 

 the classification which best suits the geology of England : 

 Geological Period. Effects noted. Localities. 



1. After the deposition Beds of argillaceous con. Derwent- Water. 

 of the Skiddaw rocks. glomcrates. 



2. During the deposition Porphyry, greenstone, Under Helvellyn, in 

 of the Snowdon rocks. and trappean conglo- Sllowdon, &c. 



merates. 



*3. After all the primary Principal elevations of Grampians, Lammer- 

 strata were deposited. primary rocks. muirs, mountains of 



Cumberland and 

 Wales. 



Conglomerate Hocks follow in the old red-sandstone. 



*4. Aflcr the deposition Immense faults, anticli- The great faults of 

 of the coal strata. nal axes, &c. Tyiicdale, the Peninc 



chain, Craven, Derby- 

 shire, Flintshire, South 

 Wales, and generally of 

 the coal districts. 



Conglomerates follow in the red-sandstone. 



5. After the oolitic pe. Unconformity of Yorkshire, Dorsetshire. 



riod. stratification between 



oolite and chalk system. 



*0. After the London Anticlinal axes and ver- Isle of Wight, Axis of 

 clay. tical strata. the Wcalden. 



At tho three epochs marked by stars, the most considerable move- 

 ments and greatest changes in physical geography appear to h;ive 

 been produced. Such changes also occurred about the same epochs 

 ou the continent of Europe : the most universal of the phenomena 

 seem to be the two earlier ones ; but it is almost impossible in any 

 case to prove that the occurrence of convulsions was synchronous at 

 distant points. Since then we can neither affirm anything with 

 respect to the change of force of the subterranean monuments at 

 different geological epochs, nor cau ascertain, except by reference to 

 the phenomena of stratification aud organic life, whether they occurred 

 more frequently in one period than another, it is impossible to draw 

 from the evidence of these disruptions any certain conclusion either 

 as to the change of the earth's proper heat or the extent of geological 

 time. If indeed the actual effects of earthquakes were to be placed 

 against tho mighty wall of the Penine fault, the vertical beds of tl 

 Isle of Wight, o the concealed dislocation of the coal-fields of Valeu- 



3 s 



