264 GEOLOGY OF THE BLACK HILLS. 



the south just below the canon, the clay-slates were observed, forming the 

 upper portion of a cliff resting unconformably on the schists at its base. 

 Both formations were, however, upturned on edge at different angles, and 

 it is possible that this apparent unconformability of the two divisions of the 

 Archaean may have been the result of a "fault" formed during the fold- 

 ing of the schists and slates. The point where this unconformability was 

 observed was almost exactly on the supposed line of contact between the 

 two formations. At short distances to the east and west the peculiar char- 

 acteristics of the rocks of the different divisions were unmistakably exposed 

 in the cliffs and outcrops of the strata. From the difference in lithological 

 character in the rocks of the two formations, the resemblance between 

 specimens of the schists and slates from the Black Hills and samples from 

 well known Laurentian and Huronian of Canada and Lake Superior, as 

 well as the observed superposition of the slates and quartzites on the tal- 

 cose and mica schists, we have been led to regard the clay-slate formation 

 as of more recent geological age than the mica-schists so extensively devel- 

 oped on the head branches of Castle, Spring, and French Creeks. This 

 subject will be found treated more fully in the section on the metamorphic 

 rocks by Mr. Newton. On examining specimens of the Archaean rocks 

 in the possession of Prof. T. S. Hunt, I find that schists from the French 

 Creek district resemble closely the " Mont Alban," or White Mountain 

 series of Professor Hunt, which he regards as older than the Huronian. 



I was informed by Mr. A. H. McKay, one of the miners who accom- 

 panied the expedition of General Custer, that Ross and himself failed to find 

 gold in prospecting on Castle Creek ; but on a small branch in the north 

 part of Elk Horn Prairie they obtained the first "color" of gold from the 

 Black Hills July 28, 1874, and during the six days the expedition remained 

 camped on French. Creek they improved the time and got gold enough to 

 prove its occurrence in the placer gravels of that stream. 



Gold was discovered on Castle Creek, above the canon, June 12, 1875, 

 by the miners assisting me in the work of prospecting. It was found in 

 small gravel deposits along the creek and in the gulches leading into the 

 stream from the south. About two or three miles below the point where 

 Custer's trail leaves the valley a bar, covering an area of two acres, gave, 



