BLACK HILLS GEOLOGY. 217 



It appears probable that the more steeply dipping strata have been 

 cut away by the erosion of the peripheral valley, leaving only the 

 horizontal portions that cap its outside border, its continuity 

 being broken to the southeast by the Elk mountain uplift. 



To sum up what has been stated, there are two series of sed- 

 imentary rocks, viz., an inner rim of Cambrian shales, and an 

 outer one of massive limestone. Both dip away from the hill 

 on those sides where they closely approach it, so that as one 

 nears the mountain one passes over the upturned edges of the 

 sedimentaries before the phonolite is encountered. On two 

 sides, namely, to the east and south, the sediments have been 

 worn away from the immediate neighborhood of the abrupt es- 

 carpment of phonolite, but they may be seen lying above ex- 

 posures of the latter rock at some little distance. On the west 

 and north the sediments extend to the very foot of the igneous 

 bluff, where they are upturned at a very high angle. On the 

 very top of the mountain a small portion of Cambrian shales 

 still remains. Bearing these facts in mind we may conclude 

 that a mass of igneous rock has found a line of weakness or 

 fracture in the vicinity of Ragged Top mountain, and has forced 

 its way through that until it has almost reached the Carbonifer- 

 ous limestone ; that it has broken irregularly across the shales, 

 so as to completely reach the limestone on the west, but has 

 preserved above itself on the north a considerable thickness of 

 shales. On the south a very small thickness of the same forma- 

 tion lies above the igneous rock. Being unable to penetrate, 

 the heavy, massive limestone, the phonolite spread out laterally, 

 forcing its way between the easily cleavable shales and sandstone 

 of the Cambrian formation, and at the same time lifting the 

 entire series of overlying rocks in the shape of a dome. But 

 as the force of the intrusion was strong, and the molten mass 

 very large, the limestone was domed up until it could no 

 longer stand the strain, a series of faults occurred, allowing the 

 fused mass to lift blocks of limestone of irregular shape and size 

 and to fill the spaces beneath them. Subsequent erosion has 

 then removed most of the limestone and revealed the irregular 

 igneous surface, from which the mountain has derived its name. 

 Annals N. Y. Acad. Sci., XII, Nov. 23, 1899 — 14. 



