206 IRVING. 



the track enters a deep gorge, which the waters of Deadwood 

 gulch have cut in a huge dike of tonahte. This dike continues 

 to form the walls of the gulch until we have almost reached 

 the mouth of Go-to-hell gulch. It is nearly fifteen hundred 

 feet in thickness and, from its great size, is somewhat more un- 

 conformable to the slates than the smaller dikes. Beyond this 

 the dikes are seen only at rare intervals, and from Central City, 

 where the Algonkian grades imperceptibly from slates and 

 phyllites into garnetiferous schists, to Deadwood, only an oc- 

 casional dike is to be seen. If, now, one ascends the northern 

 side of the gulch west of Central City, and walks thence west- 

 ward along the divide between Deadwood and Sheeptail gulches, 

 a new feature in the relations of the intrusions will become 

 manifest. Instead of the separate and distinct dikes, that ap- 

 peared in the slates along the railroad, the entire divide is made 

 up of irregular bosses of porphyries, phonolites, and intrusives 

 of all descriptions, mingled together in inextricable confusion. 

 Occasionally fragments of Cambrian quartzite and conglomer- 

 ate may be seen in the porphyry. Residual fragments of this 

 basal conglomerate may also be now and then observed lying 

 on the upturned slates, where some higher point of a former 

 Algonkian surface protrudes through the capping porphyry. 

 It seems probable that we have here the lowest horizon of hor- 



Fig. 8. Diagraiiimatic section of divide to the north of Deadwood gulch to illus- 

 trate nature of porphyry masses beneath the basal Cambrian conglomerate. 



izontal intrusion. In other words, the porphyry, which occurs 

 in dikes in the slates below, on reaching the hard basal quartzite 

 found the lines of least resistance in a horizontal direction, and 

 lifted the resisting, superincumbent mass so as to spread out on 

 the irregular surface of the Algonkian below. Dikes of phon- 

 olite seem to penetrate the older and more decomposed bosses 



