The oaks are found on all of the continents of the northern hemisphere, as well as at high altitudes just south of the equator. Their woods stand pre-eminent among those of the broadleaf series and have been highly prized from early periods. Formerly relied upon for all purposes of house and naval architecture, they did not give way to the so-called "soft woods" for houses, and to iron for vessels, until comparatively recent periods. They were supplanted for the former purposes upon the opening of the soft-wood forests of North America and of the Baltic, and for vessels subsequent to the conclusions of the American Civil War. The historical importance of oak is founded upon the reputation of the English Oak as derived from two trees, Quercus robur var. pedunculata and Quercus robur var. sessiliflora, usually taken as sub-species of Quercus robur.[12] It is said that these trees once formed large forests over Northern and Central Europe. Live-oak has always been highly esteemed, but is now very scarce. It is one of the hardest, heaviest, and most durable of constructive woods and was once largely employed in ship-building. The wood of the White Oak (_Quercus alba_) is at present preferred for most purposes for which oak is now employed, and is one of the most valuable of the American hard woods. Oak is tough, durable, easily obtained, liable to warp and check in seasoning, often hard to nail without splitting, susceptible of high polish, and not greatly liable to attack by insects. It contains gallic acid, causing peculiar taste and odor and attacking iron, the solutions staining the wood. Experiments[13] indicate that iron fastenings are shortly protected by an insoluble scale of resulting salt, and that the wood, although [p012] darkened, remains practically uninjured. The later oaken vessels were iron-fastened,[14] and cabinet-makers now employ that metal in joining oak. The barks of all species are also so charged with acid as to be used in the tanning of leather. The several kinds of oak are commercially divisible into three general groups, white oak, red or black oak, and live oak.[15] The principal species affording woods under each head are as follows: White Oak. White Oak (Q. alba). Cow Oak (Q. michauxii). Chestnut Oak (Q. prinus). Post Oak (Q. minor). Bur Oak (Q. macrocarpa). Pacific Post Oak (Q. garryanna). Red or Black Oak. Red Oak (Q. rubra). Pin Oak (Q. palustris). Spanish Oak (Q. digitata). Yellow or Black Oak (Q. velutina). Live Oak. Live Oak (Q. virginiana). California Live Oak (Q. agrifolia). Live Oak (Q. chrysolepis) [Illustration: WHITE OAK (_Quercus alba_).] [Illustration: RED OAK (_Quercus rubra_).]