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These classes are then briefly characterized, perhaps somewhat too 

 briefly to differentiate them clearly in the student's mind at this stage 

 of his knowledge of the subject. For example, the characterization of 

 contact-metamorphic deposits is insufficient to distinguish them sharply 

 from deposits of classes 4, 5, and 6. 



The next eight chapters take up one by one the several classes of 

 the scheme. They are models of clear and concise presentation. 



In the chapter on the sedimentary deposits is given an interesting 

 diagram that shows approximately what proportion of the primary ores 

 and protores of each of the various metals occur in the eight groups of 

 the genetic classification. 



Chapters 11 to 21 inclusive deal with the more general phases of 

 the subject, such as primary ore shoots, the origin of thermal metal- 

 liferous waters, and so on. The chapter on Superficial Alteration and 

 Enrichment is of course particularly good, as is to be expected of a 

 field that the author has so notably made his own. It is somewhat 

 surprising, however, to find that he has made no use of the newer 

 solubility determinations of the metallic sulphides as given in Stieglitz' 

 Qualitative Analysis or in Abegg's Handbuch der Anorganische Chemie; 

 had he used these, rather than those of Weigel, he would have found 

 less difficulty in reconciling them with the known facts of secondary 

 enrichment. 



The second half of the book (chapters 22 to 28, comprising 299 

 pages) treats of the metals and more important non-metals. The chief 

 valuable minerals of each metal and non-metal are described, chiefly 

 in their economic aspects, the genesis of the deposits is sketched, the 

 age of the deposits occurring in the United States is discussed, and 

 certain specific or distinctive features of the geology of each substance, 

 such as sulphide enrichment in the chapter on copper, are specially 

 considered. Although some economic geologists have objected to this 

 mode of treatment by substances, it is an eminently satisfactory way 

 of giving the subject concreteness to the student. As DeLaunay says, 

 the treatment by types of deposits, in spite of important advantages, 

 has the grave fault of scattering the descriptions of deposits of the same 

 substance according to a system that is necessarily somewhat hypothetical 

 and arbitrary. The general treatment of each metal or non-metal is 

 followed by descriptions of mining districts, practically all of which are 

 chosen from the United States. For some reason not clearly apparent 

 the order in which they are described conforms neither with the genetic 

 classification of the first half of the book nor with the order of their 



