326 REVIEWS 
study for interpretation. The area was land during early and middle 
Cambrian times, but toward the close of the period the sea advanced, 
covering most and perhaps all of the area. The Potsdam or Dead- 
wood sediments were largely derived from the Algonkian Crystalline 
rocks, as evidenced by the local material contained in the basal Cam- 
brian conglomerate. The Ordovician beds known in the northern 
Black Hills are not found southward, and both Silurian and Devonian 
are likewise absent. As to whether the region was land during those 
three periods or was alternately elevated and submerged, the erosion 
during the times of elevation being sufficient to remove all the sedi- 
ments deposited during the periods of subinergence, can only be con- 
jectured. The unconformity between the Cambrian and the Carbon- 
iferous is inconspicuous. 
From the early Carboniferous on through the Trias, sedimentation 
seems to have been continuous. During the Lower Carboniferous 
the region was the scene of limestone deposition in an open interior 
sea. Probably there was no considerable land mass in the vicinity 
until the Upper Carboniferous. At this time islands are believed to 
have existed in the neighborhood of the Laramie Mountains of Wyom- 
ing, and they probably furnished the clastic material of the Minnelusa 
formation. Late in the Carboniferous or early in the Permian (the 
lack of fossils in the Opeche beds prevents a definite location of the 
horizon) the waters of the area appear to have been inclosed seas, and 
to have suffered temporary reduction by evaporation in an arid climate. 
Red clays, interstratified with thin seams of gypsum, indicate such 
conditions. Mr. Darton was so fortunate as to find a few Permian 
fossils in the comparatively barren Minnekahta limestone, thereby 
showing with reasonable certainty that the Permian sea extended 
farther north and east than was previously supposed. The Spearfish 
Red Beds are classed as Triassic because of their stratigraphic relation 
to the Permian and to the Jurassic, and because of their lithologic 
resemblance to Triassic formations elsewhere in the Western United 
States. They are almost devoid of fossils. During this period, as in 
the early Permian, we may suppose that the region about the Black 
Hills possessed an arid climate and was covered by saline lakes, in 
which gypsum and red clays were deposited. 
The unconformity between the Red Beds and the marine horse 
doubtless represents an epoch of erosion during the early part of the 
Jurassic. The Sundance, Unkpapa and Beulah formations belong 
