212 - IRVING. 



are accustomed to expect in a region so seamed with intrusives. 

 It is true that on the northern side of the mountain the phonoHte 

 seems to cease almost abruptly, being complicated by the quartz- 

 porphyry, and observations in the Union Mine also show that 

 dikes and sheets of phonolite ramify in all directions through 

 the shales that lie between it and the Algonkian, and prove that 

 the lower contact of the phonolite is not nearly so regular as 

 the exposures in Whitetail and Stewart gulches would lead one 

 to believe. 



Contact metamorphism does not seem to be common either in 

 the shales above or below the mass. Siliciiication, it is true, has 

 been quite widespread, but is to be sharply distinguished from 

 contact metamorphism. It is to be attributed to the effect of 

 solutions rendered more active by the intrusion, and not neces- 

 sarily contemporaneous with it, rather than to the baking 

 effects of the heated magma. 



In closing this brief description perhaps the most striking 

 features of the intrusion are its circumscribed character, for it 

 may be readily studied in a day, and the singularly fortunate 

 way in which a rather adv^anced erosion has revealed its lacco- 

 litic nature. 



Ragged Top Laccolite. 



A little to the north of west, and about one mile distant from 

 Crown hill, the low dome-like mass of Ragged Top mountain 

 rises some four hundred feet above the level table-land of the 

 Carboniferous plateau. It lies between the tw^o confluent 

 gulches of Calamity and Jackass creeks, the former shallow 

 along the upper part of its course, but becoming precipitous as 

 it rounds the western end of the mountain. Here it unites with 

 the more deeply carv^ed gulch of Jackass creek. Thence the 

 two pass together between almost perpendicular walls of lime- 

 stone into Spearfish canon. As seen from the top of Crown 

 hill (see Plate IX.), it is a long, low, oval dome of a very regu- 

 lar aspect, and the same outline appears when it is viewed from 

 Dacy Flat to the north. It is onl}- when one ascends the mountain 

 itself that the extremelv rough and irregular character of the hill 



