412 Notices of Books. [July, 
regards matters of fadt, and in the more speculative part of the 
subjedt the author shows great discretion. Commencing with 
an account of the aim and scope of geology, he passes on to a 
brief history of its rise and progress. He next enters upon 
descriptive geology, discussing in succession — denudation, the 
destiny of the waste produced, the method of formation of 
stratified rocks, the definition and classification of derivative 
rocks, and the method of determining the physical geography of 
the earth at different periods of its past history. The volcanic, 
metamorphic, and granitic rocks are next described, followed by 
a consideration of the questions how the rocks came into the 
positions in which we now find them, and how the present sur- 
face of the ground has been produced. In all these chapters, 
whilst cultivating brevity, Prof. Green not merely gives the 
reader results, but explains very clearly the logical processes by 
which such results have been reached. In the last two chapters 
he deals with the present physical condition of the earth, the 
causes of upheaval and contortion, the origin of the heat required 
for volcanic energy and metamorphism, examining the hypothesis 
of a thin crust, and reviewing the theories of Mr. Hopkins, 
Mr. Scrope, the Rev. O. Fisher, Prof. Sterry Hunt, and Mr. R. 
Mallet. 
Mr. Green’s concluding remarks on speculative geology may 
be pronounced eminently judicious. He declares that we are 
unable to pronounce positively upon the present state of the 
earth’s interior, and that though the arguments in favour of a 
thick crust are very weighty, they are by no means conclusive. 
As to the vexed question of geological time, he points out a very 
important objection to the calculations of Sir W. Thomson, 
leading, as is well known, to a result far shorter than what 
geological and organological phenomena evidently require. Sir 
W. Thomson, starting from the basis of the Nebular hypothesis, 
assumes that the earth has been, and will be, cooling all along. 
Mr. Lockyer, however,* agreeing with the views of Prout and 
Dumas — and we may say, to a certain extent, of Mr. Ennis, as 
laid down in his work on the “ Origin of the Stars,” which will be 
noticed in our next issue — points out a very obvious method “by 
which the failing heat may have been replenished perhaps over and 
over again.” Many of the substances provisionally regarded as 
elements, merely because our resources have been so far unable 
to effedt their decomposition, may in reality be compounds 
formed from constituents which, during the early part of a star’s 
career, existed in a free state. But when the temperature of the 
heavenly body was so far reduced as no longer to keep these 
primeval elements in a free state, then combination took place, 
liberating a very considerable amount of heat. “ Thus,” Mr. 
Green continues, “ the life of a star may not have been one con- 
gee Chemical News, 0 &. 3, 1873, 
