Metamorphic rocks such as slates and schists are those changed from
igneous, sedimentary or from soils into metamorphic by pressure or heat or both. These rocks vary from dense slates in which the layers of the material are barely visible to schists in which the layers of various minerals are clearly visible and may readily split into thin plates. Because of the mode of the formation of these rocks the layers or planes rarely lie horizontal in the ground and so generally provide an unsatisfactory or poor foundation.