402 



NATURE 



\SepL 7, 1876 



represented by unconformity. Re-elevation has been accompanied 

 by disturbance of the area from a different centre than that around 

 which subsidence took place. The strata are worn obliquely, 

 and thus thickness of the mass at one place is greatly diminished, 

 though it does not follow in all cases that the maximum thick- 

 ness of the strata has been effected. 



The importance, as I deem it, the excessive importance which 

 is attached to the missing strata is asserted by biologists who, 

 apparently unconsciously, seek to gain, by prolonging the interval 

 between successive groups, the time which ought rather to be 

 sought for in tracing, were that possible, the migrations of the 

 species which seem to have suddenly died out. In other words 

 there is a reversion to the older ideas regarding the succession of 

 strata which are embodied in such phrases as the Age of Fishes, 

 the Age of Reptiles, and the like. 



But the inequality of surface which unconformity involves, 

 entails that other consequence that the maximum thicknesses of 

 the two masses of deposits do not coincide in position. Hence 

 the thickness of the strata in the area will be exaggerated, the 

 time spent in deposit also exaggerated, if the two thicknesses are 

 put together. Ihis has been done by Mr. Darwin in drawing 

 inferences from the measurements given him by Prof. Ramsay, 

 measurements which, on the face of them, do not represent a 

 continuous pile of rock. Mr. Darwin assumes either that the 

 Welsh Hills (not to speak of the Hebrides) were covered by all 

 the later strata now denuded or that if we sink a bore, say on the 

 east coast, we should go through the whole series as tabulated. 

 "When Prof Huxley took 100,000 feet as the thickness of the 

 sedimentary series, the same notion was unconsciously present, 

 the same survival of catastrophism, the onion-coat theory as 

 Herbert Spencer named it. 



The Geological Survey has corrected its tables in one im- 

 portant direction ; it has shown the contemporaneity of unlike 

 groups in different parts of Britain, the distinct types of the old 

 red sandstone, carboniferous, permian, and purbecks being 

 placed in parallel columns. To some extent this is a curtail- 

 ment of the thickness of the rock series, the dissimilar strata are 

 not piled on each other. But the curtailment might be carried 

 still farther. The marine and terrestrial conditions are simul- 

 taneous ; if we could identify the dry land for each deep sea we 

 should have possibly the overlap of periods producing extraordi- 

 nary combinations, though not perhaps of Mesozoic and Palaeozoic 

 faunas contempoia leous. But the British series may be tabulated 

 as follows : — 



Land Siirfaces. 



Lacustrine and Fluviatile. 



Cambrian. 

 Old Red Sandstone. 

 Calciferous Sandstones. 

 Coal Measures. 

 Permian. 

 Trias. 

 PurbecTc. 

 Wealden. 



Miocene. 

 Pleistocene. 



Marine. 

 Laurentian ? 

 Silurian. 



Carboniferous Limestone. 



Jurassic. 



Neocomian. 



Cretaceous. 



In the case of the Cretaceous Series, Mr. Ramsay has given 

 illustration of the ingenious views of De La Bcche regarding the 

 contemporaneity of deposits superposed one on the other. The 

 Lower Greensand is contemporaneous with part of the Chalk, so 

 were parts of the Wealden : nay, even of the Purbecks a porUon 

 must have been forming while the Cretaceous sea was gradually 

 deepening southward and eastward. 



It may be said that the recognition of the parallelism would 

 not make very much difference after all ; that it would not one 

 whit lessen the time spent in forming 500 feet of rock to know that 

 there was elsewhere another 500 feet formed at the same time. 

 But the shortening of the geological list by striking out the over- 

 laps of the formations and thus counting them only once is of 

 itself a matter of some consequence, since the maximum thick- 

 ners of the Cretaceous being nearly 3,000 feet and that of the 

 Weald 1,500 feet, even the partial coincidence, in time, of these 

 masses, would, on Mr. Croli's calculation of i foot of deposit 

 per 1,000 years, make a considerable difference in the chrono- 

 logy, still more if the Carboniferous Limestone be set against 

 its probable contemporaries the Upper Old Red Sandstone and 

 Coal-measures. Mr. Jukes' bold erasure of the Devonians was of 

 itfelf a very important change on the chronological table, and I 

 doubt not others may yet be achieved. But, it may be said, the 

 Cretaceous still rests on the Wealden ; the vertical thickness still 

 remains. But is the ordinary method of estimating the thick- 

 ness quite reliable ? In some cases, as in the productive coal- 

 measures", there is tolerable uniformity ; but among the lower 



coals and the Mesozoic strata, where the strata or groups of 

 strata are not regular, the maximum thicknesses of all are, as 

 has been already shown, apt to be taken, and^thus an aggregate 

 more or less in excess of the real thickness results. 



But recurring to an objection already referred to, arrange it 

 as you like, you get, say in Wales, a known thickness of 50,000 

 feet. But the rocks there are tilted, and the absolute depth 

 which they attain in this position is unknown. In North 

 America the Laurentians are estimated at 30,000 feet ; but 

 though there is every reason to believe that they have not been 

 covered to any extent with later deposits, the total thickness of 

 sedimentary crust is, for the same reason as in Wales, unknown. 

 B'gsby has showm how varied are the surfaces on which the 

 later deposits are laid down ; how great, therefore, must be 

 the deductions from the same total of maximum or even average 

 thickness of all formations before we approximate to the actual 

 thickness of sedimentary deposits at any one point. But take 

 the actual thickness in Wales as given in Jukes's Manual from 

 the Survey data : for the Cambrians we have from 23-28,000 

 feet ; Silurians, Upper and Lower, not counting breaks by un- 

 conformities, 20,000. If denudation takes place at the rate of 

 I foot in 6,000 years, and deposit at the same rate, we should 

 have for the Silurians alone 120,000,000 of years needed. If, 

 however, deposit takes place at the rate of I foot in 14,400 years, 

 288,000,000 millions of years would be needed for the accumu- 

 lation of the surviving strata. It is obvious that the rate of 

 deposit or denudation, or both is misunderstood. The stratified 

 rocks equal in amount the material denuded ; if we knew the 

 total amount of denudation we should know, not merely the 

 residuum of rock open to our inspection, but the total amount of 

 stratified deposits which had been formed, or at least approxi- 

 mately, for the deposit of materials removed is not synchronous 

 with their removal. Obviously these elements are not known, 

 and cannot be known to us. Mr. Croll, who has investigated 

 the question theoretically, assumes that deposit and denuda- 

 tion take place in equal times, and assumes further a uniform 

 distribution over the whole or over a part of the sea- bottom. 

 But Prof. Geikie's table shows that, if we are to take averages 

 as a safe guide, the land is lowered at the rate of two in 6,000 

 years. Moreover, if, as Mr. Croll points out, deposit was less 

 during the glacial epoch, the process must have been more rapid 

 since, and thus an irregularity is introduced which impairs the 

 value of the calculations. Prof. Hughes, in the brief abstract of his 

 Royal Institution address, which alone I have had the opportunity 

 of seeing, contests the validity of any estimates of time on the 

 basis of our existing knowledge. I do not mean to enter into 

 this question, but I may be allowed to remark that any conclu- 

 sions, founded on mean thickness of sedimentary formations are 

 of no value. It is not the time necessary for the building up of 

 a mean thickness, but that necessary for the formation of the 

 maximum thickness in particular regions which we have to 

 consider. 



If the Laurentian rocks and their equivalents are to be re- 

 garded as the earliest stratified deposits, or rather, if there is no 

 reason for believing that they were preceded by other stratified 

 rocks, the relation of Huxley's homotaxis to any classification of 

 strata having the Laurentians as a fixed point is worth investi- 

 gating. The universal diffusion of species in the earlier strata 

 was first the accepted creed of geologists. Then it was denied, 

 though the language of the earlier faith continued current. 

 Again, we return towards the doctrine of extensive simultaneous 

 diffusion, but under a very much modified form. The Chmllenger 

 reports bear testimony to the wide distribution of forms in the 

 deepest oceans, and when we turn from these and compare the 

 lists of fossil species so found widely distributed, it appears that 

 here again we have oceanic forms, or at any rate those found in 

 such limestones as are safely assigned to a deep water origin. 

 Ramsay has shown that the continental epochs in Western Europe 

 overlasted considerable periods of time. The antiquity of the 

 Atlantic and Pacific is certain ; even their primitive character 

 is possible. Thus there are two conditions — land and deep sea 

 — reasoning regarding which must be quite different from that 

 applicable to the intermediate conditions. It is exactly these 

 intermediate states which present practical and speculative diffi- 

 culty. Theories which account for mountains and oceans fail to 

 explain the " oscillations " which were wont to be appealed to 

 when terrestrial and marine surfaces succeeded each other. But 

 the assumed movement of the land is by no means a certainty, 

 and as in the kindred case of faults, we need terms which shall 

 be neutral, whether the land has moved upwards or the sea 

 shrunk downwards. The terms Palreozoic, Mesozoic, and Cain- 



