BLACK HILLS GEOLOGY. 207 



of quartz-porphyry, although specimens of the actual contacts 

 could not be obtained. Fig. 8 illustrates the character of 

 these intrusions. 



Continuing up on this divide we leave the porphyry hills and 

 again cross the Algonkian with its innumerable numbers of in- 

 truded dikes. To the north of this divide, in the direction of 

 Garden City, the entire area of Algonkian slate is one maze of 

 parallel dikes and conical porphyry hills, the former appearing 

 most frequently when the slates are cut below the level of the 

 deposition of the Cambrian, and the latter when the erosion has 

 done no more than remove the basal series and expose the 

 porphyry below. 



These conical caps to Algonkian hills have led Dr. Jenney^ 

 to suppose that erosion had removed the Cambrian from the sur- 

 face of the Algonkian previous to the intrusion of the igneous 

 rocks. This would, however, necessitate the existence of super- 

 ficial characters in the rocks. Such are invariably absent, and 

 the rocks are of typical, intrusive character. 



Besides the dikes in the gulch of Deadwood creek a great 

 number may be observed in Whitetail gulch, near Sugar Loaf 

 hill. Between that mountain and Lead City, along the Black 

 Hills and Fort Pierre Railroad, a great many dikes have been 

 exposed on the hill-side, and jut out into the gulch below the 

 track. They are of phonolite, quartz-porphyry, etc., together 

 with amphibolites of very diverse textures. 



Innumerable dikes intersect the Algonkian in the mouth of 

 Fantail and Nevada gulches. All of these form prominent 

 ridges, but must be distinguished from the ridges of quartzite 

 which are always seen in the schist areas, and by reason of their 

 indurated character often attain greater prominence than the dikes 

 themselves. 



The dikes in the Algonkian are sometimes of very great size. 

 A mass of this character occurs a short distance west of Central 

 City in Deadwood gulch. Another is the large outcrop of 

 phonolite in False Bottom creek. A third is the very large 

 dike of rock related to dacite, which is exposed on the Fremont, 



'^ Black Hills Mining Review, March 21, 1898, Vol. X., p. 10. 



