240 IRVING. 



Of the peaks discussed in Russell's paper the writer has 

 studied Crow and Terry peaks, but in addition has described 

 Sugar Loaf hill, the Needles, Ragged Top mountain. Elk 

 mountain and other igneous masses. What further study may 

 reveal in the cases of far outlying peaks, as Custer peak. Bear 

 butte, Inyan Cara, Mato Tepee and the Little Missouri 

 buttes, of course, cannot be stated, but from the fact that these 

 peripheral bodies were forced up beyond the level of the Car- 

 boniferous the conclusions adduced would seem to be equally 

 applicable to them. 



In addition to the above cited papers, a short report on the 

 geology of the same portion of the hills studied by the writer 

 appeared in the transactions of the American Institute of Mining 

 Engineers, in which mention is made of Terry peak and Sugar 

 Loaf hill as laccolites, and also of the numerous intruded sheets 

 and dikes of the district. As the paper is chiefly concerned 

 with the ore bodies, however, there is no discussion of the struc- 

 tural relations of the eruptives. 



The fact which strikes one most forcibly in the studies of those 

 who have so far written on this subject is that their conclusions 

 were the results in either case of the phenomena observed within 

 the limited district they studied. Newton's observations, from 

 the hurried nature of his survey were necessarily, confined to 

 the larger and more conspicuous eruptive bodies, and especially 

 the unique hills that lie in the periphery of the main Black 

 Hills uplift. The smaller sheets and dikes that would imply a 

 degree of fluidity inconsistent with his hypothesis were over- 

 looked by him. Even more is this true of Russell whose stu- 

 dies were made where the absence of dikes and smaller intru- 

 sions seemed phenomenal, and who was able to see only the 

 peaks farthest removed from the main eruptive center of the 

 hills. Crosby, on the other hand, made his observations directly 

 in the heart of the eruptive region, and was strongly impressed 

 by the vast numbers of small dikes, sheets and irregular bodies 

 that intersect the sedimentary rocks, and thus was led to regard 

 the intrusions as a very fluid series. 



