Troctolite (trout-stone), a variety of Gabbro (q.v.) composed almost entirely of white felspar (anorthite) and dark olivine. It is the 'Forellenstein' of the Germans.
Troctolite
Chambers's Encyclopaedia, Volume 10: Swastika to Zyrianovsk and Index, p. 301
Source scan(s): p. 0320