88 GEOLOCIY ()!• THE BLACK HILLS. 



The strata exposed are as follows : 



Archaean. 



Feet. 



1 . Arjjillaoeous slates, diijpiiig 00° west 



Potsdam. 



2. r.iowiiish yellow <;on{,'loiiierato, with quartz pebbles, resting unconformably on 



1 and (lii)piiig 2~P northeast 25 



3. lieddisli hiowii sandstone, thinly bedded at base, and alternating with soft 



shaly sandstones, containing large I'ucoids, Lingidcpis, and fragments of 

 trilobites 200 



Carho7iiferoits. 



4. Reddish brown or pinkish, ealcareous sandstone, thinly bedded, containing 



Spiri/cm camcrata, eyatliophylloid corals and erinoid columns 20 



5. Limestone ; Spirifera camerata, Prodnctiis, etc 335 



The sandstones are often riddled with small holes perpendicular to 

 the bedding, one to three or more inches long and about one-eighth of an 

 inch in diameter, often Avith rounded terminations. These are especially 

 characteristic of the white sandstones and occur more frequently in the 

 uj)per beds of the formation. Often they are so numerous that a section 

 of the rocks resembles a piece of perforated card-board. They have been 

 described by Professor Ilall* under the name of Scolithus linearis, and were 

 long thought to be the casts of a sea-weed. They are now generally con- 

 sidered the casts of worm borings, and though this explanation is more 

 acceptable, there is still room to doubt its sufficiency. It has been sug- 

 gested that they may be the peduncles of brachiopods {Lingula, etc.) or the 

 borings of the same. 



One of the most interesting points in the lithological structure of the 

 Potsdam is the occurrence of beds of quartzite both at its base and inter- 

 stratifiod with its sandstones. Near Warren Peaks, Crow Peak, and 

 Terry Peak (juartzites have been formed from sandstones by the direct 

 metamorphic action of the volcanic outbursts, and in other portions of 

 tlie IPills, particularly in the southern end of the Archaean area, there are 

 (luartzites at the base of the formation which may possibly be the result 

 of metamorphism accompanying the uplift of the Hills But in many 



* Paleoutology of Now York. Vol. 1, p. 2. 



