BLACK HILLS GEOLOGY. 205 



varieties : pre-Cambrian dikes of amphibolite and hornblendic 

 schist ; dikes of phonolite, quartz-porphyry, etc., of early Ter- 

 tiary time. 



The two are widely separated in age and petrographic char- 

 acter. The former were intruded before the metamorphism of 

 the slates and schists, and shared in their alteration ; the latter 

 came in long after those rocks had assumed a vertical position, 

 and had received their covering of sediments. The later erup- 

 tives are, therefore, to be regarded as true dikes, while the 

 earlier basic intrusions are, despite their similar structural rela- 

 tions, really intruded sheets. As both usually conform to the 

 bedding of the Algonkian rocks, and are now in a vertical po- 

 sition, we shall class them together as dikes, and defer until 

 later the discussion of their petrographic and genetic differ- 

 ences. 



The dikes of later intrusives are scattered in great profusion 

 over the entire area of the Algonkian. They may be observed 

 making long prominent hills, with a general northwest and 

 southeast trend, from the sides of which the softer and more 

 easily eroded schists and slates have been w^orn away. In Dead- 

 wood gulch their great numbers may be perhaps better appre- 

 ciated than in any other part of the area. On Dead Dog hill 

 are four large and distinct dikes striking northwest and south- 

 east, parallel to the strike of the slates. Just above Texana is 

 an exceedingly prominent one, which forms a long ridge on 

 both sides of Deadwood gulch, and which can be traced for 

 nearly half a mile in either direction. As we pass on along the 

 Fremont, Elkhorn and Missouri Valley R. R. between Texana 

 and the large dike to the west of Go-to-hell gulch, a distance of 

 hardly one mile, no less than twenty -two dikes of from ten to 

 lOO feet in width jut out from the bank along the northern 

 side of the railroad. Between these lie intervening portions of 

 schist and quartzite ; dikes of amphibolite and hornblend schist ; 

 and innumerable smaller dikes of porphyritic rock. The latter 

 are of such small size that it has been impossible to map them. 

 At the point where a large stream branches off" to the south ex- 

 tending up into the Algonkian heights beyond Deadwood gulch, 



