LETTER OF TRANSMITTAL. 



Department of the Interior, 

 United States Geological Survey, 

 Division of Geologic Correlation, 



Washington, D. C, April 1, 1891. 



Sir : I have the honor to transmit herewith a memoir by Mr. C. D. 

 Walcott, on the Cambrian of North America, prepared for publication 

 as a bulletin. 



The Division of Geologic Correlation was created for the purpose of 

 summarizing existing knowledge with reference to the geologic forma- 

 tions of North America, and especially of the United States ; of dis- 

 cussing the correlation of the formations found in different parts of the 

 country with one another, and with formations in other continents ; 

 and of discussing the principles of geologic correlation in the light of 

 American phenomena. The formations of each geologic period were 

 assigned to some student already well acquainted with them and it was 

 arranged that he should expand his knowledge by study of the litera- 

 ture, and by field examination of classic localities, and embody his 

 results in an essay. The general plan of the work has been set forth 

 on page 16 of the Ninth Annual Eeport of the Survey, and on pages 

 108 to 113 of the Tenth Annual Report, as well as in a letter of trans- 

 mittal to Bulletin No. 80 of the Survey. 



The first of the series of essays resulting from this work, prepared 

 by Prof. Henry S. Williams, pertains to the formations of the Car- 

 boniferous and Devonian periods, and constitutes Bulletin No. 80. 

 The present essay is the second of the series, and others will follow. 



Mr. Walcott finds that the American Cambrian includes three faunas, 

 of which the first and third are of continental range, and that by the 

 aid of these faunas the formations in the various geologic provinces 

 may be classified into three divisions, an upper, a middle, and a lower. 

 He finds also that these divisions may be homotaxially, and perhaps 

 chronologically, compared with divisions of the European Cambrian. 

 Yery respectfully, your obedient servant, 



G. K. Gilbert, 

 Geologist in Charge, 



Hon. J. W. Powell, 



Director U. S. Geological Survey. 



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