Sept. 7, 1876] 



NATURE 



401 



in all cases, considerable periods of time. The intercalation of 

 strata, assumed to fill up the gap, and hereby to give symmetry 

 to systematic classifications, can only be done by an appeal to 

 the statistical method, a fauna containing forms characteristic of 

 higher and lower beds being assumed to represent an intermediate 

 point in time, whereas it might be equally well claimed as re- 

 presenting an intermediate area in space, and as being possibly 

 representative of the whole gap and of some of the strata above 

 and below it. 



The definition of a formation as representing a certain period 

 of tiire, still repeated with various modifications, is to blame for 

 this and several other curiosities of procedure. But the climax 

 of symmetrical adjustments is reached when we find "natural 

 groups" established — when, in other words, an attempt is made 

 to show a regular periodicity of phenomena in geology. Dawson 

 proposed a quarternary, Hull a ternary classification, to neither 

 of which should I now refer, but that the deserved estimation of 

 these writers is apt to perpetuate what seems to be an unsafe 

 view of geological succession. 



Hull's arrangement has the merit, by force of its simplicity, of 

 bringing the vainness of the attempt into prominence. Dawson 

 has complicated his classification so as to render it impracticable. 

 A natural group of strata, one in which elevation, deep depres- 

 sion, elevation, record themselves in rocks so as to establish geo- 

 logical cycles, implies several things for which we have no 

 evidence. Most important of all does it imply, that the events 

 above noted should recur in every area in the same order, that 

 they should recur at equal intervals of time, and therefore yield 

 equal masses of strata, and above all that the superior and in- 

 ferior limits of each natural and coterminous group should consist 

 of a mass of similar strata, one portion of which shall belong to 

 the earlier, the other to the later group. Here then we have 

 implied, not catastrophic simplicity as regards the strata, but 

 something very like it as regards the subterranean forces. 



Mr. Hull has not, however, been able to surrender^imself 

 wholly to his speculation. He has admitted "gaps," breaks, 

 that is to say, for which he finds no equivalents in the British 

 series ; the strata that should occupy these gaps having been 

 either removed by denudation or never deposited, the British area 

 being at these times above water. The concession is fatal to the 

 scheme. But the very use of the word gap recalls the phrases 

 " complete and incomplete," and their nearest of kin "base of a 

 formation." Prof. Ramsay used the word " break " to mark his 

 unconformities, but no term, has been proposed for "the base of 

 a formation." The term was in constant use when such base 

 was always claimed to be a conglomerate. That notion is now 

 exploded, but no distinction is drawn between the lowest bed of 

 a group of conformable strata, and the bed or beds which repose 

 unconformably on those below them. Thus, the London basin 

 has the Thanet beds, the Reading beds, and the London clay 

 successively resting on the chalk, and each of these is the base for 

 its proper locality, unless it be asserted that in this and similar cases 

 the lowest beds once covered a wider area, and were then 

 removed. But a more important case is presented by the great 

 calcareous accumulations of the Carboniferous and Chalk series. 

 The Lower Greensand is to the latter series in England what the 

 lowest stratum of the Chalk would be if we could get at it. The 

 Carboniferous Limestone rests directly on the Red Sandstone in 

 central England, farther north it rests on the Calciferous Sand- 

 stones. Thus the base of the formation varies according to 

 locality, or rather according to the circumstances of deposition, 

 and we need a term which would indicate a difference between 

 the conformable and unconformable succession. Mr. Judd has 

 lamented the equivocal use by English writers, of the term for- 

 mation, which etymologically is as well applied to the Chalk 

 without flints as to the whole Cretaceous series. He advocates 

 "system" as applicable to the larger groups, the Cretaceous 

 system for example. But it seems as if the time were come for 

 still further restrictions of either or both terms. 



The analogy of the geological record to an incomplete volume 

 is, like most analogies, at once imperfect and misleading. Rather 

 might the record be compared to the fragments of two volumes 

 which have come to be bound together, so that it is not possible 

 to recognise the sequence. Or perhaps it might be better com- 

 pared to a universal history in which, by omission of dates, tke 

 chronology is thoroughly obscured, and the necessary treatment 

 of each nation by itself conceals the contemporaneity of events. 

 We have the aquatic record and the terrestrial record, and these 

 two are going on simultaneously. It is as yet, and probably 

 always will be impossible to recognise the marine deposits which 

 correspond to the terrestrial remains, save perhaps in the most 



recent geological times. We now know that the life of the 

 Cretaceous seas is not wholly extinct in the existing Atlantic 

 Ocean, but exists there to an extent which would entitle the 

 deposits of that area to rank by the statistical method as inter- 

 mediate between the Cretaceous and the Tertiary. It is obviously 

 impossible to include under one term deposits which are asso- 

 ciated with geographical changes so important as those commonly 

 accepted as having prevailed during the Tertiary epoch. The 

 Meosozoic forms pass gradually into the Tertiary, how gradually 

 we cannot say, since the deep sea equivalents of the European 

 Tertiaries are not certainly known to us. But as a portion 

 •survives to the t^resent day, and as, presumably, the extinction 

 was not rapid (for it is only in the case of land animals that 

 sudden disappearances are as yet probable), it is obvious that the 

 successor, the heir of the Chalk, was not the Eocene, nor 

 necessarily the Miocene known to us, but probably deposits still 

 buried under the Atlantic. 



My object is to show that, even the limitation of time which 

 Prof. Tart prescribes for us, may not after all be too narrow for 

 the processes which have resulted in our known stratigraphy. 

 Mr. Darwin speaks of the geologic record being the imperfect 

 record of the last series of changes, the indefinite extension of time 

 anterior to the earliest fossiliferous rocks being necessary for the 

 full evolution of organic forms. But is there any ground for the 

 assumption 2 True that the Laurentians contain fragments of 

 antecedent rock, but were these fossiliferous? Are they the remains 

 of land surfaces on which living baings flourished, or are they 

 only the debris of the first consolidated portion of the earth's 

 crust on which if organisms existed they may have been the most 

 primitive of our organic series ? Mr. Jukes refers to the possi- 

 bility of such earlier strata having existed, but he wrote when 

 geologists were dominated with the belief in the indefiniteness of 

 geological time. Now we are brought by physicists, like Shr 

 W. Thomson and Captain Dutton, to face the question — is there 

 evidence of such earlier masses of stratified deposits ? If we 

 allow to the physical argument all the weight to which its advocates 

 deem it entitled, if we accept fifteen millions of years, nay, even 

 if we admit one hundred millions of years as our limit, it follows 

 that we may still regard the earth as in its first stage of cooling. 

 But when we turn to the geological evidence, all that can be 

 advanced is that the Laurentian strata contain fragments pre- 

 sumably derived from earlier strata ; but metamorphosed frag- 

 ments among metauiorphic rocks ai'enot the most reliable guides, 

 and there is the positive evidence that the Laurentian area has 

 not been covered to any extent, if at all, by later deposits. So 

 far as direct proof goes, therefore, we have none that the earliest 

 known stratified rocks are not also the earliest deposited after cool- 

 ing. Even if we disregard the limiis imposed by the philosophers, 

 liberal though they are in Sir W. Thomson's hands, the absence 

 of proof that later deposits covered Laurentian areas seems 

 entitled to greater weight than is usually allowed to negative 

 evidence. At best the assertion of antecedent strata is an 

 arbitrary one, which any of us is at liberty to contradict, and in 

 favour of which no physical evidence, and only zoological pre- 

 judices can be adduced. The earliest stratified deposits known 

 are the Laurentian, and they are, so far as we know, the earliest 

 to have been deposited. 



But apart from these possible though improbable earlier 

 deposits, geological time is said to be lengthened by the missing 

 strata of later periods. Mr. Croll has given great prominence 

 to ttiis, which is another of the things taken for granted in 

 geology, commenting on Mr. Huxley's remark that if deposit 

 went on at the rate of i foot for 1,000 year.-, the 100,000 feet of 

 strata assumed by him to form the earth's crust, would be laid 

 down in the 100 millions of years which Sir W. Thomson had 

 given as the limit. But, says Mr. Croll, what of the missing 

 s'rata ? It is commonly said that we have only a part of the 

 deposits of any period, that the last have been denuded away, 

 an 1 that thus the time needed for their deposit and for their sub- 

 sequent removal are out of our knowledge. This is based on 

 what we see on the shore when the tide rises and falls and washes 

 off at each turn a part of the sand and mud laid down in the 

 interval. But the older deposits were laid down in deeper water 

 than that between tide-marks, and were for the most part laid 

 down during subsidence. Even admitting removal of part of the 

 strata to have taken place during re-emergence, the quantity so 

 withdrawn cannot be proved to represent more than a small 

 fraction of the total. To provide the needed elongatioa of 

 geological time by an appeal to arbitrary speculations is not 

 admissible. Belief on belief is, as Butler says, bad heraldry. 

 The denudation to which importance is justly ascribed is that 



