Picked up in preparation for a geology trip to the area this summer. Published as a USGS Bulletin in 1923; now reprinted by a publishing house specializing in mining history. Of interest mostly to historians. My personal exposure to mining was cleaning up afterward, so it was interesting to see what was actually involved. There’s a flow chart for the Amethyst mill; it includes a grizzly, trommels, Harz jigs, Chilean mills, and Wilfley tables; I had to look all of those up. (A grizzly is a bar grate doing initial separation of large rock chunks; a trommel is a rotating drum made of wire mesh that allows smaller material to fall through; a Harz jig uses a reciprocating piston to move material through a screen; a Chilean mill uses rotating disks to crush ore; a Wilfley table is slanted slightly and has riffles to collect heavier particles as crushed ore is washed down).

The change in geological understanding is also pretty interesting, particularly the descriptions of rhyolite. Rhyolite is a high-silica extrusive igneous rock, which forms a very viscous lava but which also erupts explosively, as tiny fragments. If the fragments are cool when they settle out of the air, the result is often called “volcanic ash”; a misnomer since nothing was burned. If they were still red-hot, they can fuse together to form “ignimbrite”, or “welded tuff”. Thus, a single eruptive layer can result in material that is so soft it can be powdered by hand pressure grading continuously to hard, glassy rock that rings like a bell when you hit it with a rock hammer. This issue is further complicated because ash-fall tuff can become cemented by sedimentary processes, just like sandstone, and thus result in a reasonably solid rock. This wasn’t understood in 1923, and the authors (William Emmons and Esper Lawson) assume all the competent rhyolite they encountered resulted from lava flows rather than ignimbrite deposition. The clues were there; the eruptions of Mt. Pelee in 1902 and Lassen Peak in 1915 had produced ignimbrites but on a much smaller scale than the ones around Creede and the dots weren’t connected until the 1960s.

Unfortunately this reprint is missing the geologic map included in the original, which makes it difficult to compare changes in rock unit names. Interesting for the geological history, nevertheless. ( )