PETROGRAPHY OF SOME LAMPROPHYRIC DIKE ROCKS 

 OF THE COEUR D'ALENE MINING DISTRICT, IDAHO. 



By Earl V. Shannon, 

 Assistant Curator, Department of Geology, United States National Museum. 



INTRODUCTION. 



Most of the occurrences upon which the following descriptions are 

 based were examined by the writer in and around the mines of the 

 Coeur d'Alene District in the summer of 1917. Ransome and Cal- 

 kins '^ had done much work in the region, mapping the dikes in a 

 single color and stating in their text that certain of the rocks were 

 most properly called minettes, while others were nearer kersantite. 

 Later Dr. Joseph B. Umpleby of the United States Geological Sur- 

 vey turned over to the Museum all specimens collected by him in 

 the course of his field work in this region. Inasmuch as other 

 duties have necessitated that publication of results by Doctor 

 Umpleby be indefinitely postponed, descriptions of his material are 

 included in the present paper. There had already been deposited 

 in the United States National Museum the specimens of these rocks 

 collected by Mr. F. C, Calkins in his work in the Coeur d'Alene Re- 

 gion, and permission was kindly extended by him to reexamine these 

 and to redescribe such as might be of interest in connection with the 

 present work. There were thus available specimens from a large 

 proportion of the dikes which occur in the producing area. The 

 descriptions are based entirely upon museum material. 



While the following paper contains much rather monotonously 

 descriptive matter, it relates to a group of igneous rocks of un- 

 common interest. The observations recorded are made as complete 

 as possible for the reason that a great number of the dikes which 

 occur associated with important ore-bodies have been made visible 

 only in mine workings, and the great majority of mines in which 

 they occur have been exhausted and abandoned and their workings 

 rendered permanently inaccessible. Thus the student of this particu- 

 lar district may find here recorded evidence which is no longer ob- 

 tainable in the field. 



These black ferromagnesian rocks have been locally designated by 

 various names as syenite, diorite, diabase, etc. The rock from the 



iRansome, F. L., and Calkins, F. C, Prof. Paper, U. S. Geol. Survey, No. 62, 1908. 



Proceedings U. S. National Museum, Vol. 57— No. 2318. 



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