October 27, 1921] 



NATURE 



2S3 



importance entitles them. In our own country we 

 are familiar with them in the Jurassic system, but 

 with us this system is far from attaining its maxi- 

 mum thickness — it does not exceed 8000 ft. — 

 while elsewhere it is represented by deposits of 

 20,000 ft. or more. The presence of numerous 

 and well-marked disconformities in the British 

 Jurassic rocks is, therefore, not surprising; 

 whether they have the same importance in areas 

 of maximum deposition has yet to be shown. 



The estimates based on the rate at which 

 sodium is supplied by rivers to the sea are in 

 remarkable agreement with those derived from a 

 study of straptified deposits. The objection that 

 most of the sodium in river water has been directly 

 derived from the sea was raised long ago by Mr. 

 Ackroyd, of Halifax, but was shown on investiga- 

 tion to be invalid. 



Xo importance can be attached to the salinity 

 of the sea in the early part of the Cambrian epoch, 

 for as much time or more had elapsed before that 

 period as followed after it. The first era of geo- 

 logical time, which has been called the Protaeon, 

 and the second, or Deuteraeon, are of approxi- 

 mately equal length. From what we know of the 

 behaviour of existing marine forms when exposed 

 to brackish water conditions we have no reason 

 to suppose that the Cambrian faunas could not 



have flourished in a sea only half as salt as the 

 existing ocean. 



Juvenile waters, often rich in sodium and 

 chlorine, no doubt contribute to the contents of 

 existing rivers, but if, as seems likely, they fur- 

 nished a larger contribution in past times, the 

 effect would be to shorten instead of lengthening 

 Prof. Joly's estimate. 



Finally, it may be pointed out that in the only 

 instance where estimates based on the thickness 

 of deposits can be brought into comparison with 

 a stricter determination of time the former have 

 been found in excess. This stricter determina- 

 tion is due to Baron de Geer, who, by counting 

 the number of annual layers of sediment left 

 behind by the great ice-sheet in its retreat, found 

 for the duration of post-glacial time a period of 

 12,000 vears, and thus shorter by several thou- 

 sand years than those arrived at from a study 

 of the post-glacial deltas in the Swiss lakes. 



Geologists are not greatly concerned over the 

 period which physicists may concede to them ; 

 they do not much care whether it is long or — in 

 moderation — short, but they do desire to make 

 reasonably certain that it is one which they can 

 safely trust before committing themselves to the 

 reconstruction of their science, should that prove 

 to be necessary. 



By Prof. J. \\'. Gregory, F.R.S. 



THE claim that geological time must be 

 restricted within a score, or a few score, 

 million years was regarded by most geologists 

 with incredulity, since a score million years was 

 of little more use to geology than the seven days 

 of the Pentateuch. Now that physical evidence 

 allows the age of the earth to be counted by the 

 thousand million years the problem is of less 

 concern to the geologist, except from the 

 hope that the uranium-lead ratio may fix geo- 

 logical dates in years, and from the interest of 

 reconciling the conflicting results of the different 

 methods. 



The geological estimates to which most weight 

 has been attached are based on the saltness of the 

 sea. The salinity argument has been widely 

 accepted as sound in principle ; the estimates 

 varied from 70 to 150 million years, and some 

 intermediate length was regarded as inevitable. 

 Allowances were made for various factors ; but 

 they added only a few per cent! to the total, and 

 did not multiply it by ten or more. 



The validity of the salinity argument may be 

 tested by two checks — the supply of chlorine, and 

 the denudation required to account for the amount 

 of sodium ; and as shown by Dr. A. Holmes, each 

 of these indicates a much longer period than the 

 sodium. 



The supply of chlorine in igneous rocks is quite 

 inadequate to convert their sodium into chloride. 

 Most of the sodium chloride in river water is 

 probably marine in origin, and only the sodium 

 in the bicarbonate and sulphate is a fresh addition 



XO. 2713, VOL. 108] 



to the sea. On this ground the salinity estimate 

 should be approximately doubled. Again, to 

 obtain all the sodium in the sea from igneous rocks 

 would involve the denudation of improbable 

 volumes of them, and, at the rate usually accepted, 

 the age of the earth should be multiplied three- 

 or four-fold. 



The fundamental objections to the salinity argu- 

 ment are against (i) its assumption that the sea 

 was originally fresh, which palaeontological evi- 

 dence renders improbable : the oldest fauna, the 

 Cambrian, has the characteristics of a marine 

 fauna, and the contrast between the freshwater 

 and marine faunas was as sharp in Palaeozoic 

 times as it is to-day ; (2) its omission to allow for 

 the large supplies of sodium chloride raised from 

 beneath the earth's surface by magmatic waters ; 

 (3) its assumption of uniform denudation. The 

 earth has probably undergone deformations that 

 led to alternate periods of quick and slow crustal 

 movement ; during the times of repose the surface 

 would have been planed down and rivers would 

 have become sluggish and denudation slow. As the 

 earth is now under the influence of a time of quick 

 movement, denudation is faster than the average. 

 A multiplication of the earth's age five-fold for 

 this difference would not be excessive. 



During quick crustal movement volcanic action 

 would be more powerful, the discharge of hydro- 

 chloric acid and sodium in hot springs would be 

 increased ; and as denudation is now acting on 

 land in which sodium chloride has been produced 

 in unusual quantities by volcanic action the esti- 



