208 IRVING. 



Elkhorn and Missouri Valley Railroad, at the apex of the east- 

 erly pointing loop between Texana and Bald Mountain. Of 

 these the phonolite in False Bottom creek is the most con- 

 spicuous. It is a very coarse, trachytoid variety, and is mingled 

 with masses of a more fine-grained character. The mass is 

 best exposed about 300 feet north of the junction between the 

 False Bottom and Carbonate roads. As one stands on the sum- 

 mit of War Eagle hill, and looks north, this mass can be seen to 

 cover a great area, and may be easily distinguished from the other 

 intrusions and dark mica slates, by the whitish decomposition 

 product that coats its exposed surfaces. The contact with the 

 slates is not very regular, as it sometimes cuts athwart them 

 and presents a quite uncomformable boundary. 



In the bed of Squaw creek at the mouth of Labrador gulch 

 and thence on down the stream, a great irregular mass of am- 

 phibolite is exposed. This is unconformably covered by the 

 Cambrian, and is extremely interesting, in that it shows that the 

 older basic eruptives intruded in the Pre-Cambrian sediments, 

 were at times exceedingly irregular, and of great extent. For 

 the most part they seem to have been intruded sheets, which 

 have since been turned on end and buried by the Cambrian ; 

 but this Squaw creek mass as well as other gabbroic amphi- 

 bolites between Deadwood and Custer Peak, and many others 

 in the northern hills, would seem to point to the existence of 

 large intruded laccolitic masses of pre-Cambrian age. One can- 

 not fail to be impressed with the extent of these metamorphosed 

 eruptives, for they show that the hills were the seat of a period 

 of prolonged and widespread igneous activity, long before the 

 deposition of the Cambrian. 



Such dikes of the later eruptives as occur in these amphibo- 

 lite areas, do not, of course, preserve the regularity of strike 

 which characterizes those in the slates ; for in general there is 

 no cleavage in the massive rocks to determine their direction. 



The dikes in the Algonkian occur in such great profusion 

 that it has been impossible in one season to trace out even the 

 larger ones. The have not therefore been indicated on the map 

 except along the course of Deadwood gulch, and a few in the 



