I'liEFACE. XIII 



the eruptive rocks of the Eureka District, appears as an appendix to this 

 report. It presents the results of a careful examination of several hundred 

 thin sections prepared from a large number of rocks, representing every 

 variety known to occur in the region. It is a concise statement of results of 

 a systematic study of the material and is of great interest, bearing directly 

 upon many geological questions connected with eruptive masses. Mr. 

 Iddiiigs's report is illustrated by six plates, four of which are reproductions 

 of photomicrographs, showing some interesting features in structure of fine 

 groundrnass, and two of drawings of minute crystals and microscopic objects 

 found in the rocks. At the time these photomicrographs were produced 

 they were superior to anything which had been done in this class of illus- 

 tration. 



Mr. Walcott's report upon the "Paleontology of the Eureka District" 

 was published as Monograph VIII of the U. S. Geological Survey, in 1884. 

 It gives the results of a detailed study of the organic forms obtained 

 throughout a wide range of geological formations, the region having proved 

 an exceptionally rich one in paleontological material from Cambrian, Devo- 

 nian and Carboniferous rocks. In addition to the descriptions of many 

 forms new to science and the identification of over five hundred species, 

 the report contains notes, more or less full, upon many species which 

 presented in their characters or geographical distribution information not 

 heretofore published. The work is illustrated by over five hundred and fifty 

 accurate drawings of fossils, arranged on twenty-four plates. Four plates 

 represent the fauna of the Cambrian, two that of the Silurian, ten that of 

 the Devonian, and eight that of the Carboniferous. All specific identifica- 

 tions of organic forms from Eureka referred to in this work were made 

 by Mr. Walcott. 



After the completion of the field work for the Eureka map, Mr. J. S. 

 Curtis began his investigations of the ore deposits found on Ruby Hill. 

 The surface maps published by Mr. Curtis were taken from the atlas sheets 

 accompanying this monograph. Mr. Curtis's report appeared in 1884 as 

 Monograph VII of the U. S. Geological Survey, and is entitled "Silver-Lead 

 Deposits of Eureka, Nevada." It is a valuable work and one which forms 

 an important part of the scientific memoirs relating to the Eureka District. 



