BLACK HILLS GEOLOGY. 227 



ing on the shale in Annie creek, some fifty feet below. As we 

 follow it around the series of little gulches running into Squaw 

 creek we find it overlying the heavy basal quartzite of the Cam- 

 brian in a perpendicular cliff, and apparently running around 

 the hill, to connect with the large quartz -porphyry mass of 

 Gushurst hill. The twofold character of the porphyry mass 

 will be understood if we descend the bed of Squaw creek from 

 the old Portland mill. Here we find the pinkish-brown " Bird's 

 Eye " quartz-porphyry in sharp contact with a deep bluish 

 looking rock which makes the sides of the gorge, until we ar- 

 rive at the large amphibolite exposures far down the stream. 

 This contact may be traced up on Gushurst hill to the north- 

 east and in the opposite direction up the bed of the gulch for 

 some distance until obscured by talus. The blackish porphyry 

 forms a very irregular intrusion and has tilted up the basal 

 Cambrian conglomerates on the west at an angle of nearly 

 twenty degrees. This westwardly dipping rim of quartzite can 

 be traced from the head of Squaw creek far around out on the 

 divide between Squaw creek and Labrador gulch. 



The two varieties of rock when examined under the micro- 

 scope present little or no difference, except that the feldspars of 

 the darker type are prevailingly square in outline and those of 

 the other generally rounded. The blackish quartz-porphyry of 

 this lower mass underlies the quartzite completely down to 

 Squaw creek, and may be seen again in Labrador gulch. In 

 this vicinity its relations are obscured by so complicated a maze 

 of intrusions that it would be a hopeless task to try to unravel 

 them. The more important facts with regard to the intrusions 

 will, however, be readily grasped, i. c, that there are here two 

 separate quartz-porphyry masses, one of which has been in- 

 truded partly beneath the Cambrian and partly above it. 



The quartz-porphyry of the lower intrusion is cut by two 

 tinguaite dikes, one in the bed of Squaw creek and of a rather 

 fined-grained character ; the other a very coarse dike (described 

 on page 259) which cuts the porphyry both above and below the 

 quartzite, and of which many huge boulders have fallen into 

 the bed of Squaw creek. This dike is very persistent, and can 

 be traced for a distance of nearly one mile. 



