88 GEOLOGY OF THE BLACK HILLS. 



The strata exposed are as follows : 



Archaean. 



Feet. 



1. Argillaceous slates, dipping 60° west 



Potsdam. 



2. Brownish yellow conglomerate, with quartz pebbles, resting unconformably on 



1 and dipping 25° northeast 25 



3. Keddish brown sandstone, thinly bedded at base, and alternating with soft 



shaly sandstones, containing large fucoids, Lingulepis, and fragments of 

 trilobites 200 



Carboniferous. 



4. Beddish brown or pinkish, calcareous sandstone, thinly bedded, containing 



Spirifera earner ata, cyathophylloid corals and crinoid columns 20 



5. Limestone ; Spirifera camerata, Productus, 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 with rounded terminations. These are especially 

 characteristic of the white sandstones and occur more frequently in the 

 upper 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 Hall* 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- 

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

 Terry Peak quartzites have been formed from sandstones by the direct 

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

 the Hills, particularly in the southern end of the Archaean area, there are 

 quartzites at the base of the formation which may possibly be the result 

 of metamorphism accompanying the uplift of the Hills. But in many 



* Paleontology of New York. Vol. 1, p. 2. 



