REVIEWS 491 



GEOLOGY 



Manas a Geological Agent. By R. L. Sherlock, D.Sc, A.R.C.Sc, F.G.S. 

 [372 pp.] (London : H. F. & G. Witherby, 1922. Price 205. net.) 



There is nothing intrinsic to the science of geology to preclude the study 

 of the part played by man in the development of the earth's crust, but it 

 has been tacitly agreed to term this action artificial and dismiss it in as few 

 words as possible. The author points out that a difference of degree rather 

 than kind can be considered to exist between man's part and that of other 

 rock-forming animals in support of this statement. However, there is a 

 difference in that man's work is, or tends to be, always progressive, and new 

 problems of bridling Nature are continually being solved. The book is 

 chiefly descriptive, but a few words to refute the suggestion of man's opposi- 

 tion to natural forces border on the philosophical. 



Quality, quantity, and permanence are the three chief factors discussed. 

 Man is a rock-destroyer and a rock-builder, but more far-reaching are the 

 results of his work in altering drainage and even climate. As a rock- 

 destroyer, man has brought up from mines vast quantities of material, 

 incidentally affecting the surface by subsidence, while his activities in 

 excavation at the surface are also familiar to all. There is something more 

 interesting, however, in the accumulation of artificial products which the 

 author compares with natural rocks in a chapter sure to appeal to every 

 type of reader. Man's interference with drainage and sea coast is shown 

 to have both a positive and negative effect on erosion, with a possible net 

 result of surface gain. Deforestation, improved drainage, and increased 

 carbon dioxide production all tend to raise the temperature, and similar 

 arguments might be employed to explain former geological changes of 

 temperature. 



The author has taken much trouble to estimate the amount of man's 

 activity, but the order of magnitude is all we can hope for or indeed require. 

 The methods of obtaining statistics are both interesting and instructive, 

 the figures used being those for London and England, suitable and repre- 

 sentative localities. The results, dependent chiefly on population and 

 engineering industries, show man's action to be irregular, much greater than 

 natural changes and most resembling glacier action. 



Finally, since the direction of human action is constantly changing, 

 man tends to destroy his own work. " Man's most permanent memorial 

 is a rubbish-heap," says the author, referring, of course, to the made-ground 

 forming a city's foundation, " and even that is doomed to be obliterated." 



Apart from its interest as a geological work, the subject is one of economic 

 and philosophical value, and the book can be well recommended to the 

 general reader ; the middle course between the scientific and popular treat- 

 ment has been adopted with advantage. All have read of the influence 

 of environment on man, but the converse forms a novel and interesting 

 study. 



Edgar D. Mountain. 



Earth Evolution and its Facial Expression. By W. H. Hobbs. [Pp. xvii -f 

 178, with 6 plates and 84 figures.] (New York: The Macmillan Co., 

 1921. Price 15s. net.) 



Prof. Hobbs has added another to his fascinating expositions of geologica 

 science in his book on Earth Evolution and its Facial Expression. He starts 

 from the view that since the planetesimal hypothesis of the origin of the earth 

 has now largely replaced the nebular hypothesis of Kant and Laplace, the 

 foundations of geology need to be rebuilt in accordance with the new ideas. 

 The book deals with certain fundamental problems of theoretical geology, 

 connected with the growth of continents and mountains, and the impress 



