Continent Europe and Asia together as a single continent, dubbed "Eurasia". In this model, the world is divided into six continents, with North America and South America considered separate continents. Geologists use the term "continent" in a different manner from geographers. In geology a continent is defined by continental crust: a platform of metamorphic and igneous rock, largely of granitic composition. Some geologists restrict the term 'continent' to portions of the crust built around stable Precambrian "shield", typically 1.5 to 3.8 billion years old, called a craton. The craton itself is an accretionary complex of ancient mobile belts (mountain belts) from
Middle-earth and despite the additional expense, work and delays involved. The definitive map of Middle-earth was published in "The Lord of the Rings". This iconic map incorporates all the chief locations of the main story, and also includes places from some of the background histories. This map was also published (with various additions and embellishment) as a stand-alone poster: "A Map of Middle-Earth". It is important to note that this map does not cover all of Middle-earth, but only shows the north-west of the continent. This is made more explicit in the revised version published in "Unfinished Tales", which is titled