It is the purpose of this paper to suggest the hypothesis that the Homestake ore body is in the main a replacement deposit in a calcareous slate series, and that it owes its position to the presence of a strong fault and its form to its replacement character and to structural factors.

These conclusions are not stated as final, as further study of this problem is contemplated. Paleozoic and Tertiary rocks cover and igneous intrusions in places obscure observations, while the examination underground was very incomplete.

The Black Hills are an oval-shaped mountainous tract, trending nearly north, on the extreme western edge of South Dakota. Physiographically this mountainous tract is the dissected core of an oval-shaped domical uplift which involves rocks as late in age as Tertiary.

Erosion has stripped the cover from this oval dome and etched the underlying basement series of pre-Cambrian schists.

It is with this . . .

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