Reviews 



The Economic Resources of the Northern Black Hills. By J. D. 

 Irving. With Contributions by S. F. Emmons and T. A. 

 Jaggar, Jr. (Professional Paper No. 26, U. S. Geological 

 Survey, 1904.) Pp. 222. 20 plates, 16 figures. 



This report treats of the general geology and the economic or mining 

 geology of an area of about six hundred square miles on the northeast 

 flank of the great central dome of the Black Hills. 



The essential structural features are: (i) the great central dome, now 

 very greatly reduced and dissected by erosion, of laccolithic granite; (2) 

 a border of greatly metamorphosed Algonkian sediments, which have been 

 compressed into isoclinal folds, and whose strike is, in this district, north- 

 westerly; (3) a great series of Paleozoic, Mesozoic, and Cenozoic sediments 

 which dip away to the northward from the central igneous core. The 

 central granite mass gave off great intrusive tongues which cut the Algonkian 

 schists in all directions. Numerous later intrusions of porphyry are found 

 in nearly all the formations from Algonkian to Benton. 



On pp. 20 and 21 is to be found an excellent feature of this paper, 

 namely, a summary, in tabulated form, of the essential points of interest 

 in connection with each formation. Besides giving the usual facts needed 

 for correlation, there is an additional column which is devoted to the 

 essential topographic characteristics of each formation. From an examina- 

 tion of this, one can readily grasp how important a means in correlation 

 the topographical element may become. 



The chief interest in this region lies, of course, in the mineral wealth, 

 and it is this phase of the subject to which the major part of the report is 

 devoted. The main productive mining district is included in an area of 

 about one hundred square miles, extending from the town of Perry, on 

 Elk Creek, northwestward to the town of Carbonate, on the east bank of 

 Spearfish Canyon, with its widest and most productive portion between 

 Terry Peak on the southwest, and Gorden on the northeast. In the neigh- 

 borhood of Terry Peak there has been the greatest igneous activity and 

 also the greatest ore deposition. 



The ore deposits are classified as follows : (a) those in the Algonkian ; 



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