proceedings: geological society 487 



obstructed, ])ut found relief in an overthrust fault in the limestone to 

 the west. The differential horizontal movement resulted in shearing 

 along the plane of the cross fault, which is nearly vertical, and passes 

 into the thrust fault to the west. 



Discussion: A. C. Spencer remarked that it was to be regretted 

 that the later Triassic sediments concealed the relations so that the 

 extension of the fault to the eastward could not be traced. 



Sidney Paige asked whether the cross-cutting portion of the fault 

 was necessarily vertical. Might it not have a rather shallow dip? 

 Stose explained the evidence which pointed to a steep dip. D. F. 

 Hewett inquired whether there was any indication of thickening of 

 strata on the limbs of the folds which had been described. Stose 

 replied that there was no evidence of this. Sidney Paige asked re- 

 garding the evidence of faulting and as to the presence of igneous 

 masses to whose intrusion the compression and folding could be ascribed. 

 Stose cited a number of features which gave evidence of the fault. 

 Regarding intrusions, he said that there was nothing of the kind near 

 enough to attribute the compression to this cause. R. B. Sosman 

 inquired whether the injections of Triassic basalts showed any con- 

 nection with the fault. Stose replied that none was shown. 



C. N. Fenner, Secretary. 



The 296th meeting was held in the lecture room of the Cosmos Club 

 on April 28, 1915. 



informal communications 



Sidney Paige: A model illustrating character of faulting at the 

 Homestake ore-body. At a previous meeting of the Society the 

 speaker had suggested an hypothesis to explain the origin of the Home- 

 stake Ore-Body of Lead, South Dakota. At that time it was pointed 

 out that stratigraphic work on the pre-Cambrian had shown the pres- 

 ence of a fault, on the two sides of which the schist series had divergent 

 strikes, and that the series on the eastern side terminated at this fault 

 line. It was also shown that dolomitic limestones and their impure 

 schistose equivalents formed an important member of the series and 

 occupied precisely the outcrops of the Homestake Ore-Body along the 

 fault line. The folded character of these beds was pointed out and 

 the significance of these folds in determining the shape of the ore-body 

 at various levels within the mine was emphasized. To portray this 

 relationship a plaster model has been constructed and a wooden copy 

 made. Very simple structural assumptions (based on field obser- 

 vations) were made in constructing this model. An anticlinal fold was 

 modeled in wax; on the main fold minor folds were imposed much 

 after the fashion of innumerable instances observed in the field. This 

 fold was given a pitch, was cut at an oblique angle by a fault, and was 

 penetrated by horizontal mine levels. The results are shown in the 

 model. Considering that the solutions which formed the ore-body 

 rose along the fault-plane and permeated the calcareous series for a 



