8 COLORADO FERBEKITE AKD THE WOLFRAMITE SERIES. 



CHARACTERISTICS OF THE FERBERITE. 



Ferberite occurs in the Boulder County field not only in large 

 quantity, but in places in clean, beautifully developed crystals, and 

 very commonly in small crystals coated with foreign material. 

 The ferberite found over most of the field is jet black, but in a few 

 places it is brown. It is opaque even in thin sections prepared for 

 microscopic examination, and the clean crystals and cleavage faces 

 are generally lustrous black. 



The ferberite of Boulder County is very resistant to weathering 

 and forms placers which have been successfully worked. The fer- 

 berite of the Black Hills (see analysis 81, pp. 32-33) decays, leaving 

 a skeleton of hydrous iron oxide, and wolframites that are close to 

 ferberite in composition decay similarly, but no alteration has been 

 noted in the ferberite of the Boulder field, although it is in many 

 places coated and discolored by hydrous iron oxide and other minerals. 



The other physical characteristics accord fairly well with those 

 given by Dana 1 for the wolframites. The cleavage along 6 is per- 

 fect, and in some specimens a parting is shown parallel to a, but I 

 have not noticed the parting parallel to t (102) mentioned by Dana. 

 The fracture ^uneven and the mineral is brittle. Its streak and 

 powder are dark brown nearly black and its hardness is about 5. 

 Its specific gravity j as determined on selected crystals, is 7.J9J). 



A splinter of pure material fuses to a globule which has a crystalline 

 surface and is not magnetic. The brown mineral from the Rogers 

 tract is almost infusible before the blowpipe, and is strongly mag- 

 netic after heating. 



Although ferberite is not very soluble in acids, if it is finely pow- 

 dered and boiled a few minutes with concentrated hydrochloric acid 

 it gives a solution which, on the addition of zinc, readily shows a 

 characteristic blue color, followed by the almost equally character- 

 istic violet and brown. 



GEOGRAPHY AND GEOLOGY OF THE fcOTJLDER 



DISTRICT. 



Nederland, the commercial town of the Boulder tungsten field, 

 lies 2 miles east of Cardinal station, on the Denver, Boulder & West- 

 ern Railroad, and 4 miles north of Rollins ville, on the Denver, 

 Northwestern & Pacific Railway. 



The ferberite-bearing area lies on an elevated plateau, above 

 which, on the west, rise massive and imposing peaks. The altitude 

 at Nede^and is 8 f 237 feet. On the east an abrupt scarp descends 

 from the plateau to the Great Plains. Streams draining the plateau 

 have cut deep canyons extending back from the scarp to distances 

 determined by the abrading power of the streams. 



i Dana, E. S., System of mineralogy, 6th ed., p. 983, 1909. 



