It was, in its essence, an ultraneptunian theory of the Earth, and was to a large extent based on field geology discerned during trips throughout Egypt and other Mediterranean countries. It is based essentially on processes today known as sedimentation, excluding all other geological or geomorphological agents except some minor aspects of weathering. The work, therefore, seems modern as it touches sedimentation, but fantastical when touching other fields. De Maillet observed, but did not always understand, all the major types of rocks forming the Earth's crust.