188 IRVING. 



Rhyolite family. 



5. Quartz-porphyry 275 



Andesite family. 



6. Mica-diorite-porphyry 281 



Dacite family. 



7. Dacite 284 



Diorite family. 



8. Tonalite 286 



Basalt family. 



9. Augite vogesite 287 



Of pre- Cambrian age. 



10. Amphibolites 288 



B. Theoretical discussion 289 



V. ORE-BODIES 294 



A. Algonkian 295 



B. Cambrian 296 



C. Carboniferous 311 



D. Placers 314 



E. Acknowledgments 315 



I. INTRODUCTION. 



The Black Hills of South Dakota, as has been so well set 

 forth by Newton in his classic work on the region and by many 

 subsequent writers, constitute an elevated area, roughly ellip- 



f^ N r A N A I 



i 



IV V M 



R A S K A 



Fig. 5. Diagram to show location of Black Hills. 



tical in outline, situated on the western border of the state of 

 South Dakota, but extending also into Wyoming. See Fig. 5. 



