Shergottites are igneous rocks of mafic to ultramafic lithology. The dominant lithology of this unit is mudstone with some interbedded sandstones. Sequences of sedimentary and volcanic rocks are subdivided on the basis of their lithology. Unconsolidated surficial materials may also be given a lithology. Facies determined by lithology are called lithofacies; facies determined by fossils are biofacies. The lithology underlying the dam is a metamorphic rock called gneiss. Converted-wave analysis is also used to determine subsurface lithology and porosity. This helps improve discrimination between lithology, porosity and fluid effects. Thus differences in lithology, grain size, and preexisting fabrics determine a different rheological response. Ontario, owing to its size, has diverse geology that varies in structure, age, and lithology. The similar lithology extends beneath the city in a tectonic window. Strictly, formations cannot be defined by any criteria other than primary lithology. The proportions of bridgmanite and calcium perovskite depends on the overall lithology and bulk composition. Systems are unrelated to lithostratigraphy, which subdivides rock layers on their lithology. The lithology of a rock is a description of its readily visible physical characteristics.