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Text File  |  1992-09-02  |  382b  |  11 lines
  1. An igneous rock formed below the Earth's
  2. surface, a form of basalt, containing
  3. relatively little silica (basic in
  4. composition). Dolerite is a medium-grained
  5. (hypabyssal) basalt and forms in minor
  6. intrusions, such as dykes, which cut across
  7. the rock strata, and sills, which push
  8. between beds of sedimentary rock. When
  9. exposed at the surface, dolerite weathers
  10. into spherical lumps.
  11.