UNTIL the end of 1918, Boulder County, Colorado, ranked as one of the principal tungsten-producing d~stricts of the world, but in August, 1919, there was practically no production, due to decline in demand, decrease in price, and large importations. The tungsten in this district occurs almost entirely as ferberite, although some hi~bnerite is found. In the early development of this district, the ores were concentrated in stamp mills designed for gold ores. Present milling plractice is by stage crushing and screening, each reduction is followed by concentration by means of devices best adapted to each size. As a rule the first concentration is by jigs, either of the Harz type or Richards pulsating type, the feed varies from 4 to 20 mesh, averaging IO mesh. Jig railings are reground by rolls, and after screening may be treated in " fine " jigs of the launder type, yielding 2o-mesh product, or may be treated on " coarse" tables. Table tailings are reground and prepared for the slime tables ; table middlings may be reground and passed into a middling circuit or held for separate treatment.
Crushing is carried to such an extent that when the tailings are discharged, practically all of the mineral has been liberated. A large proportion of the tails will pass a Ioo-mesh screen. The ore is first crushed to ~ to I inch with jaw crushers, rolls are used in preparing the mill feed. Ball mills are used for regrinding, and in one mill stamp.s are used to regrind the mill railings. Within the past few years stamp,s have been abandoned for fine crushing because of excessive sliming, and jigs introduced to, save the coarse mineral. Canvas tables are installed in all the mills, except one, and their use is said to result in a recovery of about IO per cent. of the total, although they make a low-grade product which must be tabled.
Extractions vary from 75 to 9 ยฐ per cent., according to grade * Communicated by the Director.