228 Reviews — The Principles of Economic Geology. 



that most of his groups grade into each other by imperceptible 

 transitions. The whole question really hinges on the extent to which 

 the circulation of meteoric waters has taken part in the processes 

 of ore-formation, and as is generally known more importance is 

 assigned to this agency in America than in Europe, perhaps mainly 

 owing to the influence of Posepny. In many American writings 

 there are still strong evidences of the survival of the lateral secretion 

 theory of Sandberger, and indeed something very like it is generally 

 put forward in explanation of the origin of the lead-zinc deposits 

 of Missouri and Kansas. Still more common is the conception of 

 vein-formation by downward movement of meteoric waters into 

 a region of high temperature where metals are dissolved and carried 

 to a higher level by subsequent rise of the heated solutions. 

 Acceptance of this theory is rendered difficult, amongst other 

 evidence, by the fact that many deep mines are dry in their lower 

 levels. The common occurrence of distinct ore-zones around and 

 above intrusions is also difficult to explain on this hypothesis. These 

 questions are discussed at length by Professor Emmons, who gives 

 a most interesting summary of the divergent views on the subject, 

 with a statement of the arguments for and against each of them. 



A whole chapter is devoted to a detailed discussion of the super- 

 ficial alteration and secondary enrichment of ore-deposits, so 

 important from the practical point of view. Although the general 

 underlying principles are well understood, certain points are still 

 in need of explanation ; it is still somewhat uncertain whether 

 chalcocite and some other sulphides are ever truly primary, although 

 the balance of evidence from observation suggests that they are. 

 The whole process of oxidation, solution, and reprecipitation 

 obviously depends on the chemical and physical conditions 

 prevailing in the different regions of the groimd-water, but the 

 chemical changes involved are very complex and not yet fully 

 elucidated. 



The second and larger part of the book consists of descriptions of 

 actual occurrences of minerals of economic value. These are very 

 clear and effective, but it is perhaps admissible to remark that in 

 this section a certain want of balance is shown. Descriptions are 

 given of many American deposits of little economic value and still 

 less scientific interest, while the important mining regions of other 

 parts of the world receive scant attention. It may be suggested 

 with all due deference that the object in view would have been 

 better attained by treatment of a smaller number of examples, 

 culled from all parts of the world and selected so as to illustrate 

 general principles. Thus the student of economic geology, for whom 

 the book is primarily intended, would acquire a better sense of 

 proportion and learn to appreciate more fully the scientific and 

 commercial value of certain highly important types of mineral 

 deposit which do not occur in North America. 



R. H. Eastall. 



