STRUCTURE 297 



The most important member or members within this series of rocks, 

 economically speaking, are the limestones. 4 They may be regarded as a 

 part of a calcareous slate series and grade downward, through a calcare- 

 ous slaty phase, into very thin-bedded siliceous black slates. Their lower 

 limits are thus ill-defined. Their upper limit, fortunately, lies only a 

 short distance stratigraphically below the "lower" conglomerate. These 

 limestones are best exposed at the junction of Deadwood Creek and 

 Blacktail Gulch, particularly in the wagon-road cut descending from 

 Terraville, where it turns westward to descend into Deadwood Creek. 

 Fine exposures are likewise to be found in the railroad cut just east of 

 the mouth of Bobtail Gulch, and also in Bobtail Gulch on its eastern side 

 below the lower stamp-mill. 



The top of these limestones 5 is believed to pass from Deadwood Gulch, 

 just below Blacktail Gulch, south westward to a point about 1,800 ± feet 

 south of the Terraville bridge, where the series is cut off by the fault. 

 The base of the series is not so readily located. Impure limestone may be 

 found on the wagon road in Terraville at the upper stamp-mill, and the 

 lean ore exposed in the De Smet cut, forming a perfect narrow synclinal 

 trough, proves to be a decidedly calcareous or dolomitic rock. 6 Calcare- 

 ous beds in the slate series were also observed in the town of Central, 

 nearly opposite Sawpit Gulch, and calcareous slates were found still far- 

 ther west behind the mine hoist. 



It is in this region (near Central), at the base of the limestone series, 

 that the rule of eastward dip described above does not apply, for a short 

 distance east of the probable location of the fault-plane westward dips 

 may be seen, and from here down to Blacktail Gulch the rocks seem to be 

 thrown into a low, much disturbed anticline, which, combined with the 

 transitional character of the limestone series, makes it doubly difficult to 

 establish a basal boundary. 



Notwithstanding this obscurity, the fact remains that a calcareous 

 series of rock strikes at an angle into and is cut off by a belt of quartzites, 

 and this at the apex of the main Homestake ore body. This discordance 

 of strike demands the presence of a fault. The top of the limestone series 

 may be regarded for stratigraphic purposes as just below the "lower" 

 conglomerate, and the "upper" conglomerate lies only about 350 feet 

 above the "lower." It is of particular interest, therefore, to trace or pick 

 up these conglomeratic beds along their strike to the southwest. It is 

 believed that both of these horizons have been recoamized in the Caledonia 



* The word "limestone" is used in all parts of this paper as a convenient term to ex- 

 press the idea that carbonate rocks are involved. Dolomitic limestones are present. 

 "Paleozoic cover, Tertiary cover, porphyry intrusions, and float, all Obscure relations 

 "Specimen examined microscopically was obtained south of Terraville bridge. 



