No. 120: Nov-Dec 1998
The Black Hills of the Dakotas are composed of Precambrian shists that have been intruded by granite. The Harley Park granite is well known to us all because it has been carved into the monumental visages of four presidents at Mount Rushmore. Nature, too, has expressed herself on a giant scale nearby.
"Around the granite, the shists are host to numerous spectacular pegmatites. These were mined and quarried in the early years of this century for both the large sheets of mica and also the spoduomene, which was a valuable source of lithium. The largest crystals were of the spoduomene, which were found up to 20 m [63 feet] long. Looking like great white tree trunks with two cleavages along their length, they were affectionately known as spod logs."
(Waltham, Tony; "Spod Logs," Geology Today, 13:207, 1997.)
Comment. Any crystal 63 feet long is worthy of mention in this newsletter!