<P> Sills parallel beds (layers) and foliations in the surrounding country rock . They can be originally emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills into near vertical orientations . Sills can be confused with solidified lava flows; however, there are several differences between them . Intruded sills will show partial melting and incorporation of the surrounding country rock . On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow . In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere . Because sills generally form at shallow depths (up to many kilometers) below the surface, the pressure of overlying rock prevents this from happening much, if at all . Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not . </P> <P> Certain layered intrusions are a variety of sill that often contain important ore deposits . Precambrian examples include the Bushveld, Insizwa and the Great Dyke complexes of southern Africa, the Duluth intrusive complex of the Superior District, and the Stillwater igneous complex of the United States . Phanerozoic examples are usually smaller and include the Rùm peridotite complex of Scotland and the Skaergaard igneous complex of east Greenland . These intrusions often contain concentrations of gold, platinum, chromium and other rare elements . </P> <P> Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by relatively short dike - like segments . Such sills are known as transgressive, examples include the Whin Sill and sills within the Karoo basin . The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic reflection data . Such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive . </P>

Can a layer of basalt be a sill