AGE OF THE EARTH — RAYLEIGH. 257 



but with us this system is far from attaining its maximum thick- 

 ness — it does not exceed 8,000 feet — while elsewhere it is represented 

 by deposits of 20,000 feet or more. The presence of numerous and 

 well-marked disconformities in the British Jurassic rocks is, there- 

 fore, 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 stratified 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 in- 

 vestigation to be invalid. 



No 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 Protseon, and the second, or 

 Deuteraeon, are of approximately equal length. From what we know 

 of the behavior 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 exist- 

 ing ocean. 



Juvenile waters, often rich in sodium and chlorine, no doubt con- 

 tribute to the contents of existing rivers, but if, as seems likely, they 

 furnished a larger contribution in past times, the effect would be 

 to shorten instead of lengthening Professor Joly's estimate. 



Finally, it may be pointed out that in the only instance where esti- 

 mates based on the thickness of deposits can be brought into com- 

 parison with a stricter determination of time the former have been 

 found in excess. This stricter determination is due to Baron de 

 Geer, who, bjr 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 years, and thus shorter by 

 several thousand 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 reason- 

 ably certain that it is one which they can safely trust before commit- 

 ting themselves to the reconstruction of their science, should that 

 prove to be necessary. 



By Prof. J. W. Gbegory. 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 



