262 GEOLOGY OF THE BLACK HILLS. 



The limestone is almost horizontal in its bedding, and the stream grad- 

 ually flitting deeper through the strata as it descends finally exposes the 

 Potsdam sandstone at its base, resting unconformably on the upturned 

 edges of schistose rocks. Here the lower layers of the Potsdam are seen 

 to be a coarse yellow or brown conglomerate, closely filled with pebbles 

 of white quartz, in places nearly uniform in size, from 1 to 2 inches in 

 diameter, worn smooth and round by the action of water. The schisto.se 

 rocks are exposed in great variety, dipping first west as we descend the 

 stream and then east at high angles. The strike is in a general north and 

 south to northwest and southeast direction. 



The rocks of this section evidently belong to the first division of the 

 Archaean, and can be traced south across the park countiy to the French 

 Creek district, where they are interlaminated with dikes of feldspar granite; 

 but no granite is found in this vicinity. The schists are often very mica- 

 ceous, although a gray talcose quartz-schist, having a slate-like structure, 

 appears to be the prevailing rock, occurring in broad belts interstratified 

 with mica-schist. These rocks, though distinct in bedding, insensibly 

 merge into each other in character ; mica-schist, from a preponderance of 

 mica, becoming a soft, easily-decomposing rock, often highly garnetiferous, 

 while with a greater proportion of quartz it i)asses into a fine-grained mica- 

 schist, or ultimately to a quartzite with only traces of mica. 



Chlorite schist and. talcose schist are extensively developed on some 

 of the branches of this stream, and strata frequently are found completely 

 filled with small red garnet crystals often not more than one-thirty-second 

 of an inch in diameter. Quartz occurs as segregated veins, usually not con- 

 tinuous for any great distance, locally expanding into bunches and irregular 

 masses of quartz, following the stratification of the schists, but not crossing 

 the bedding. These irregular dejDOsits of quartz were very abundantly 

 distributed through the rocks ; often strings of lenticular masses of quartz 

 followed a line of lamination in the schists, or thin veins could be traced for 

 short distances coinciding with the stratification. They were not, however, 

 true continuous fissure veins, but irregular segregations formed during the 

 folding and metamorphism of the strata. The quartz is usually milk white 

 and crystalline, sometimes vitreous, transparent, and stained and coated at 



