XII PEEFACE. 



allotted time except for the untiring energy and interest of all those coiuiected 

 with the survey. In the geological work I was fortunate in having the 

 cooperation of two thoroughly equipped assistants, both of whom have since 

 attained honorable distinction by published writings in their special lines of 

 research. To Mr. Charles D. Walcott was assigned the collection of the 

 paleontological material, while Mr. Joseph P. Iddiugs was engaged in woi'k- 

 iug among both volcanic and sedimentary rocks. 



The report appears in two parts, one a volume of text, the other an 

 accompanying atlas of topographical and geological maps and cross sec- 

 tions, and as the text is, in great measm-e, explanatory of the atlas, the two 

 can be considered only as parts of the same work. 



A paper embodying the more important results obtained at Eureka 

 was prepared in 1882 and published in the Third Annual Report of the 

 Survey as an abstract of the final monograph. It was accompanied by 

 a geological map similar to sheet iv of the atlas. The volume of atlas 

 plates bears the imprint of 1883, but is now issued in complete form for 

 the first time. In its more essential features the present report was pre- 

 pared several years ago, but the completion of the manuscript has been 

 delayed from time to time for various unforeseen reasons, mainly b)^ press- 

 ure of other duties. It presents, as concisely as is consistent with clearness 

 and completeness, the principal geological facts gathered in the field and 

 such general deductions as have been di-awn from their study. I have 

 endeavored to make each chapter complete in itself, and this has necessitated 

 the repetition of certain observations, as a large number of facts are more or 

 less related to the subjects discussed in the different chapters. It is an 

 advantage, however, to the special reader, to have such facts as he may 

 need brought together under one groiiping, and not to feel obliged to 

 search through the volume for them. 



The atlas consists of thirteen sheets. The preparation of the topo- 

 graphical map was intrusted to Mr. F. A. Clark, who employed three able 

 assistants in the field — Mr. G. H. Wilson, assistant topographer with the 

 plane table; Mr. Gr. Olivio Newman, m charge of triangulation, and Mr. 

 Morris Bien, assistant topographer. 



A special paper by Mr. Iddings, upon the microscopical petrography of 



