172 Revieics — Prof. John Perry — The Age of the Earth. 



only in the uppermost beds, on the borders of the Devonian, to 

 which formation also belong the by far greater number of theso 

 fossils which have been discovered in England, Spitzbergen, etc. 

 The Gotland forms, on the other hand, lie deeper, in one of the oldest 

 beds of Gotland — the widely extended marl shales which appear 

 nearly all over the island below the limestone. There is no 

 possibility that the marl shales of the east coast differ stratigraphi- 

 cally from those of the west coast, however unlike they may be 

 palseontologically. A description of the dissimilar faunas which, 

 these shales contain has already been given by rae^ some years 

 since. Up to the present no fossil fish has been discovered in the 

 newest strata of Gotland. These from Lau are therefore the oldest 

 known fish from the entire Upper Silurian in so far as the marl 

 shales are homotaxial with the Wenlock beds. But still further — 

 Since it has been found that the Conodonts in the Cambrian Green- 

 sand of St. Petersburg are not fish remains, and as those from North 

 America described as Lower Silurian are completely Devonian 

 in type, and from their position in disturbed beds cannot be con- 

 sidered with any degree of possibility as true Lower Silurian, we 

 must regard the Gotland Cyathaspis not only as the oldest known 

 fish but also as the oldest vertebrate, which, until a further discovery 

 is made from still older beds, lends to it an importance which it 

 would not otherwise have possessed. G. J. H. 



IL — On the Age of the Earth. By Prof. John Peiiry, F.E.S. 

 "Nature" (January 3rd, 1895), vol. li. pp. 224-227. 



OF the many attempts to determine limits to the earth's age, 

 one of the best known is that derived by Lord Kelvin from 

 the cooling of the earth. In order to reduce the problem to the 

 simple form required by mathematical analysis, Lord Kelvin was 

 obliged to make several assumptions. He imagined the earth to be 

 a homogeneous solid mass, with the same conductivity and other 

 heat-properties as the surface rock. He supposed it to have been 

 initially at a uniform temperature of 7000° F. throughout, and that 

 its surface was suddenly brought to, and kept at, the temperature 0°. 

 And, lastly, though aware that there might be a small loss of 

 potential or chemical energy, he assumed the earth to be losing 

 energy only in the form of sensible heat. Then, from the known 

 average rate at which the temperature increases with the depth, he 

 obtained 100 million years as a probable value of the earth's age, 

 and 20 million and 400 million years as the limits between which 

 it must certainly lie. 



The value of Lord Kelvin's estimates of course depends entirely 

 on the closeness with which the natural and assumed conditions 

 agree, and Pi'of. Perry points out that, in taking the conductivity 

 uniform throughout the earth, the two sets of conditions are in 

 reality made to differ very widely. It is not unreasonable to 



1 Ueber die Scliichtenfolge des Silur auf der Insel Gotland. Neues Jahrb. 1888, 

 Bd. i. p. 147. 



