476 PROCEEDINGS OF THE NATIONAL MVSEVM. vol.57. 



Hecla dike is locally called " black porphyry," a not inappropriate 

 name. The present studies seem to indicate that the hornblendic 

 and the biotitic rocks form two fairly distinct classes and inter- 

 mediate rocks are somewhat rare. Spessartites are somewhat more 

 abundant than minettes and kersantites and vogesites are quite ex- 

 ceptional. 



GENERAL OUTLINE OF THE GEOLOGY OP THE REGION. 



The chief aim of the original field work was to endeavor to develop 

 some logical theory in regard to the genetic relation of the lampro- 

 phyric rocks to the ore-bearing veins of the district and to the 

 monzonitic rocks which are presumed to have supplied the ore-de- 

 positing solutions. 



The areal and economic geology of the region have been described 

 in detail by Ransome and Calkins, and it is necessary liere only to 

 mention in bare outline the broader features of the areal geology. 

 The district occupies a portion of the large area of pre-Cambrian 

 rocks which covers the greater portion of northern Idaho and north- 

 western Montana. These sediments, which are of Algonkian age, 

 comprise a vast thickness of beds known collectively as the Belt 

 Terrane. In the Coeur d'Alene District these rocks were divided by 

 Calkins into the following members, the divisions being based upon 

 lithologic characters : 



Formation. 



Thielmess. 



Striped Peak; red and green flaggy sandstones and shales, top eroded. 



Wallace; mainly calcareous shale with some limestone 



St. Regis; green and purple shales and sandstones 



Revett; pure white thick-bedded quartzites 



Burke; greenish sericitic slates and thin-bedded sericitic quartzitep... 

 Prichard; blue-black slates quartzites and argillites, base not exposed. 



Total 



Feet. 

 1,000 

 4,000 

 1,000 

 1,200 

 2,000 

 8,000 



17. 200 



The important ore bodies occur mainly in the lower formations, 

 the Prichard, Burke, and Revett, a condition which is probably in 

 some measure influenced by the lithologic character of the forma- 

 tions but is apparently more because, at the time of deposition of the 

 ores, the rocks of the higher formations in very few cases were in 

 a position to be reached by the ore-bearing solutions before their 

 load of dissolved minerals had been exhausted. Mo'^t of the areas 

 occupied by the lower formations as exposed at the present surface 

 occupy islandic horsts which are partly surrounded by graben of 

 the higher formations which have been faulted down by gigantic 

 post-mineral faults. If commercial ore-bodies exist, in these depressed 



