Reviews 



Fourth Biennial Report o] the State Geological Survey of North Dakota. 

 By A. G. Leonard, State Geologist, E. J. Babcock, and C. H. 

 Clapp. 312 pp., 37 pis., map. Grand Forks, 1906. 



This volume continues the systematic descriptions of the economic pos- 

 sibilities of the state which have been published in previous years. North 

 Dakota possesses extensive deposits of high-grade clays, as was well shown 

 in the exhibits of the School of Mines at the St. Louis and Portland Exposi- 

 tions. The report contains chapters on the origin, chemistry, value, and 

 physical properties of clays in general, and on the stratigraphy and economic 

 geology of North Dakota clays in particular, together with a description 

 of the methods of mining and manufacture now employed. H. H. 



The Production of Gold and Silver in igo6. By Waldemar Lind- 



GREN, and Others. Advance chapter from Mineral Resources for 



1906. U. S. Geol. Surv. 265 pp. Washington, 1907. 



The total of these two metals mined in the United States, amounting 



in value to $132,630,200, showed an increase of over $10,000,000 for the 



year. Colorado, Alaska, and California were the chief producers of gold, 



and Montana, Colorado, and Utah of silver. H. H. 



Iowa Geological Survey. Vol. XVII. Annual Report for 1906. 

 588 pp., 62 pis., 44 figs. Des Moines, 1907. 

 The major portion of this volume consists of a description of the quarry 

 products of Iowa. Special stress is laid on cements and cement materials, 

 and the careful general treatment of the properties, uses, and preparation of 

 different grades of cement will be found valuable by economic geologists, 

 and those commercially interested, in all parts of the country. H. H. 



The Grenville-Hastings Unconformity. By Willet G. Miller and 

 Cyril W. Knight. An extract from the Sixteenth Report of 

 the Ontario Bureau of Mines, 1907. Part I, pp. 221-23. 

 The authors find themselves unable to agree with certain of the con- 

 clusions of the International Committee of 1906 in regard to southeastern 



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