478 PROCEEDINGS OF THE NATIONAL MVSEVM. vol. 57. 



4. Continued upward advance of the magma column, giving the 

 Coeur d'Alene batholith as it at present exists. 



6. Slight subsidence of the region and intrusion of the lampro- 

 phyric dikes. 



6. Beginning of long-continued subsidence, giving gi*eat struc- 

 tural faults. 



As suggested by Stewart^ the fact that the rocks of the Coeur 

 d'Alene intrusion present some essential differences from the main 

 mass of granitic rock in Idaho indicates that some differentiation had 

 probably taken place before it was intruded. 



THE LAMPROPHYRE DIKES. 



The lamprophyric dike rocks are widely distributed in the Coeur 

 d'Alene region as well as in a large part of the surrounding territory. 

 They are most abundant in tlie Prichard formation, are common in 

 the lower Burke, and are increasingly rare higher in the series. Those 

 which have reported from the formations above the Burke are 

 doubtful, as they are thoroughly decomposed and may belong to the 

 diabases rather than to the lamprophyres. 



As is well known the group of rocks called lamprophyres belongs 

 to the class known as diaschistic or complementary instrusives and 

 results from the tendency of a magma of medium composition to 

 separate into two components by the process of gravitative differen- 

 tiation, a mafic division containing the majority of the ferromag- 

 nesian minerals, and represented by lamprophyres, and a felsic divi- 

 sion consisting of the more acidic light-colored constituents of the 

 rock and represented by aplites in the case of a normal granite or 

 granodiorite. The presence of lamprophyric rocks in wide distribu- 

 tion over the Coeur d'Alene region constitutes the strongest argu- 

 ment in support of the assumption that the region is underlain by a 

 granitic batholith. Yet from a batholith of the composition of the 

 granitic rock of central Idaho aplites should be expected to form at 

 least twice as much of the volume of the complementary rocks as 

 lamprophyres. Aplites are not definitely known to occur in the Coeur 

 d'Alene district, although Hershey has mentioned some very much 

 decomposed dikes which may originally have been of aplitic char- 

 acter. As to wh}' lamprophyres should be so abundantly represented 

 while aplites are almost, or quite, absent, the explanation probably 

 lies in the well-recognized fact that mafic magmas are fluid at much 

 lower temperatures than are acidic magmas. It is probable that the 

 lamprophyres did not extend much above the level of the present 

 erosion surface and that they were enabled to reach their present 

 position because they were fluid at temperatures below the crystal- 

 lizing point of aplites which solidified at much greater depth. The 



' .Touroal of Geology, vol. 22, No. 7, p. 684. 



