BLACK HILLS GEOLOGY. 221 



B, & M. R. R. has revealed the character of peak. Extremely- 

 deep cuts have been made through the ridges that jut out in 

 that direction, and in them are exposed Cambrian shales and 

 sandstones lying perfectly horizontal. Vertical dikes of a com- 

 pact, fine-grained, white quartz-porphyry cut the sediments, but 

 no large masses of igneous rock appear to disturb their hor- 

 izontality. At Terry Station a large dike of quartz-porphyry 

 (different from the rock at the summit of the mountain) is seen 

 on the lower side of the railroad extending down into Fantail 

 gulch. More fine-grained dikes of quartz-porphyry of a verti- 

 cal character also appear on the side of the railroad at this 

 point. 



As we stand at Terry Station and look southeast along the 

 railroad an exceedingly conspicuous perpendicular wall of 

 quartz-aegirite-porphyry can be seen, just above the bend in the 

 roadbed, which curves in around the head of Stewart gulch. 

 (Plate XV.) The rock may be traced northwest along the rail- 

 road almost to Terry Station, and in the opposite direction to 

 within half a mile of Aztec. At both of these localities it gives 

 place to horizontal Cambrian strata, but its relations to the 

 shales were not clearly made out. Again, this mass may be 

 traced far up (half a mile) into the densely wooded slope of the 

 second great southeastern outlier of the peaks, and from prom- 

 inent points within this thicket still other outcrops of the same 

 rock maybe seen between. This exposure is probably one of a 

 series of comparatively thin sheets which, with their partings of 

 Cambrian strata, constitute the Terry Peak mass. 



The only portion of the mountain of which the writer was 

 unable to make examination is the series of ridges and peaks 

 which connect it with Deer Mountain to the southwest ; but 

 from the descriptions of Newton, and from the appearance of 

 the great phonolite masses lying to the west of Englewood, it 

 seems probable that in that direction a great complication of 

 intrusions exists. 



There are still two more exposures that seem to throw light 

 on the geological character of the peak ; the Snowstorm and 

 Sunset shafts. Concerning the first. Professor F. C. Smith 



