furruginous

Presumably originally a misspelling of ferruginous, that remained either in error or in minority, mainly technical, use from about early 19th century to mid-20th century.

furruginous (comparative more furruginous, superlative most furruginous)

1. Alternative form of ferruginous (“containing iron”)
   • 1804 Antoine François comte de Fourcroy: A general system of chemical knowledge

     The quartzose iron is somewhat difficult of treatment, nevertheless it does not resist the long-continued action of acids any more than furruginous sands and the different species of ochres.

   • 1804 Anthony Florian Madinger Willich: The Domestic Encyclopaedia; Or, A Dictionary of Facts

     The natural coral seems to receive its colour from iron, as spirit of vitriol acquires from it a furruginous taste; and on calcination, some particles are discoverable among the ashes, that are attracted by the magnet.

   • 1839 John Beck: Beck's, late Fairfax's, Leamington guide

     In advancing nearer the surface, the fluid thus impregnated meets with a stratum of porus iron-stone, which is combined with the silica, and which, in digging Mr. Smart's well, has been found to exist under the town; and hence the furruginous bi-silicate.

   • 1907 Ernest F. Burchard: Illinois State Geological Survey — Yearbook for 1907

     There are a few ledges of partly consolidated gravel conglomerate, and locally near the base of the cut is 4 to 6 feet of sand that has been indurated by a dark furruginous cement, forming a sandstone.

2. Alternative form of ferruginous (“rust-coloured”)
   • 1823 William Scoresby: Journal of a Voyage to the Northern Whale-fishery

     The caterpillar feeds on the Kola; it is grey, with alternate rows of white and furruginous spines; the pupa yellowish, variegated with black.