No. 2318. PETROGRAPHY OF DIKE ROCKS OF IDAHO— SHANNON . 477 



blocks they have been dropped downward far beyond the reach of 

 mining exploration. 



The igneous rocks are present in small amount. Chief among 

 them is a group of granitic rocks ranging from syenite to quartz- 

 monzonite or quartz-monzonite prophyry in composition, which occurs 

 as a number of small areas which presumably are upward projec- 

 tions of a single small batholith. The intrusion is very long as 

 compared with its width and trends in a northeasterly direction. 

 Calkins states that its exposure is probably the axis of a very old 

 flat anticline which has largely been obliterated by later deforma- 

 tion. This monzonitic intrusion has been regarded by Ransome and 

 others as the most probable source of the metalliferous ores of the 

 district. Hershey^ discredits the theory that the ore minerals were 

 derived from the igneous rock and applies a modification of the 

 older lateral-secretion theory which implies that the lead and zinc 

 were derived by leaching from a certain narrow horizon in the upper 

 Prichard formation in which these metals were present in some form 

 as an original clastic constituent. In his latest paper, which, un- 

 fortunately, is generally inaccessible to the majority of persons who 

 might be interested in the subject, Mr. Hershey contends that the 

 ores are older than the Coeur d'Alene monzonite intrusions, and fur- 

 ther that the contact-metamorphism of the Ninemile area is dis- 

 tinctly older than the intrusive rock at present exposed and like- 

 wise older than the ores which the metamorphic silicates inclose. 

 His evidence seems to, in part, substantiate these latter conclusions, 

 although his views as to the source of the metallic minerals have 

 met with no general acceptance even upon consideration of his com- 

 plete evidence. 



Ransome believed that the Coeur d'Alene batholith was an upward 

 projection of the continuation of the Central Idaho batholith which 

 he assumed to underlie the entire district. In part from the modified 

 conclusions of both Ransome and Hershey, we may derive the fol- 

 lowing succession of events : 



1. Intrusion of the central Idaho batholith, extending under the 

 Coeur d'Alene region to connect with the Priest Lake batholith to 

 the north, probably with an upward extension beneath the center of 

 the Coeur d'Alene area. 



2. A renewed upward advance of the column of magma under the 

 central portion of the Coeur d'Alene district, giving off emanations 

 which produced intense local metamorphism in the overlying rocks. 



3. Deposition of the ores by solutions given off by the underlying 

 igneous rock of the Coeur d'Alene. 



1 Genesis of lead-silver ores in Wardner District, Idaho. Min. and Sci. Press, June 1, 8, 

 and 15, 1912. Ori^ of Lead, Zinc, and SilTer in the Coeur d'Alene, Min. and Sci. Press, 

 Sept. 27 and Oct. 4, 1913. Origin and Distril)ution of Ore in the Coeur d'Alene, Private 

 Pub., San Francisco, 1916. 



