98 GEOLOGY OF THE BLACK HILLS. 



of . well developed casts of Polythalmia, miuute mollusks, and branching tnbuli, and 

 that the same variety of the petrifying material is found as in fossil casts, some being 

 well defined greensand, others reddish, brownish, or almost white. 



Bailey was the first to announce that other forms beside the Polythal- 

 mia produce greensand. Principal Dawson* calls attention to the fact that 

 glauconite is akin to materials found filling- the pores of Pre-Silurian fossils, 

 such as the serpentine in the cells of the Eozoon of the Laurentian 

 formation. 



As to the origin of the greensand our knowledge is still deficient. 

 Whether it is a compound secreted or formed by animal interposition, or 

 whether it is a fossilizing mineral deposited by a purely chemical process 

 we have no facts at present to determine. 



To complete the observations on the formation, the line of its outcrop 

 around the margin of the Archaean area will be rapidly passed over, begin- 

 ning at Castle Creek where it was first seen and noting its appearance at 

 the points where it was particularly studied. The Carboniferous limestone 

 plateau which borders the slates on the west has been cut into by the 

 numerous valleys of the creeks draining eastward, and each valley is a 

 canon with natural geological sections in its walls. The lower portions of 

 the Carboniferous stand in cliffs and beneath them are imperfect exposures 

 of the Potsdam and the schists. The last increase in prominence as we go 

 eastward down the streams until beyond the serrated and broken edge of 

 the plateau they alone are exposed. 



The limestone always forms bold bluffs 100 or 200 feet in height, and 

 sometimes stands in isolated buttes or peaks some distance in front of its 

 main escarpment. From the base of the limestone cliff a broad and gentle 

 slope descends toward the valleys, and the Potsdam, from its soft and friable 

 nature, is generally concealed under a grassy slope or covered by limestone 

 debris, so that cliffs or exposures of the sandstone are not common and a 

 complete and good outcrop of the Potsdam is but seldom seen. 



On Castle Creek the limestone lines both sides of the valley for eight 

 miles or more in castellated bluffs, standing with their summits 500 or GOO 

 feet above the stream. Though the valley in its lower part is deeply cut in 



* The story of the Earth and Man. J. W. Dawson, 1873. p. 229. 



