BLACK HILLS GEOLOGY. 209 



neighborhood of Sugar Loaf hill. But were the whole of the 

 Algonkian area carefully mapped, it is no exaggeration to say 

 that at least one-third if not more, would be igneous rock. 

 Some idea of this may be obtained from the map if we conceive 

 the dikes on Deadwood gulch to be produced in either direction 

 to the limits of the slate exposures, and the space between them 

 seamed with dikes too small to map. 



b. Intrusions in the Cambrian. 



I. Laccolites. 



Of the bewildering series of igneous intrusions that intersect 

 the Cambrian formation the most conspicuous, and without 

 doubt the most important and interesting, are the laccolitic 

 peaks. As compared with those of the Henry mountains, as 

 well as those described by Whitman Cross from Colorado, they 

 are of a much smaller size, a fact which has made their study a 

 matter of comparative simplicity. Within the area .mapped are 

 no less than six igneous masses of a distinct laccolitic character, 

 in addition to which are described two laccolites from the vicinity 

 of Nigger hill to the west of Spearfish creek. With the ex- 

 ception of Crow Peak, of which the writer has made only a 

 cursory examination, the two most perfect laccolitic masses are 

 Sugar Loaf hill and Ragged Top mountain, and these will for 

 this reason be given the precedence in description. 



Sugar Loaf Hill Laccolite. 



Sugar Loaf hill is situated just to the east of Whitetail 

 gulch, almost directly opposite to the mouth of Stewart gulch. 

 It forms a sharp peak, which, when compared with Terry Peak 

 and the general level of the surrounding country, attains only 

 an insignificant elevation (6,030 feet). The waters of Whitetail 

 creek have carved around its western and northern sides a deep 

 gorge, from which the mountain rises in an abrupt, almost pre- 

 cipitous cliff to the height of 550 feet. On the east and south 

 from the summit is a steep fall of 100 feet, to the comparatively 



