230 IRVING. 



portions. Ascending higher on the side of Squaw creek we 

 encounter the hornblende-mica-diorite-porphyry of the Red- 

 path laccolite, which here has spread out into thinner sheets. 



Again, not far above the mouth of Squaw creek, two sheets, 

 one of mica-diorite-porphyry, the other of a fine grained phon- 

 ohte, may be observed with a very thin parting between them, 

 dipping with the Cambrian toward the west. Besides these 

 larger masses many smaller sheets, innumerable dikes and ir- 

 regular masses occur on Squaw creek. In the vicinity of Rich- 

 mond hill and Ragged butte, two large dikes of quartz-porphyry 

 jut out toward the south, and form exceedingly conspicuous 

 land marks. 



Between Crown Hill and Twin Peaks, near the Crown mine, 

 is a sheet of light green segirite-quartz-porphyry, similar to 

 that of Elk mountain. 



Still farther is a prominent knoll formed of a projecting out- 

 crop of coarse trachytoid phonolite of a yellowish color, and 

 not to be distinguished megascopically from a trachyte. Pass- 

 ing down the divide to the northwest, we next encounter lime- 

 stone, and finally the two-fold conical hill, which has been 

 called Twin Peaks. It is composed of a coarse, rotten quartz- 

 porphyry which forms two rather sharp, conical peaks connected 

 by a somewhat lower ridge of the same rock. On the south and 

 west, 2IO feet below the summit, we find limestone, but a little 

 farther around toward the west a prospect hole shows the Cam- 

 brian shales, and the same formation is to be found east and 

 northwest. On the west it seems to come in contact with the 

 mica-diorite-porphyry. The tinguaite dike occurring in the 

 Ulster mine can be traced up almost to the summit of the hill 

 by fragmental outcrops, and probably cuts the eruptive forming 

 the hill. No disturbance of the surrounding sediments was 

 observed. The hill is probably the remnant of a laccolite in- 

 truded below the limestone of the Carboniferous, and from 

 which erosion has removed all of the sedimentary covering and 

 left only the core of the hill resting in apparent conformity 

 upon the Cambrian shales. 



Beyond the mouth of Squaw creek, in Spearfish caiion, about 



