(1) Any barrier to the upward migration of petroleum, allowing it to accumulate. (2) Any dark colored extrusive igneous rock. A reference to the tendency of basalt and similar rocks to form columnar joints
Iowa State University, College of Liberal Arts and Sciences, Department of Geological and Atmospheric Sciences, Glossary of Geologic Terms (based on the glossary in Earth: An Introduction to Geologic Change, by S. Judson and S.M. Richardson, Englewood Cliffs, NJ, Prentice Hall, 1995), Iowa, USA 2004. http://www.ge-at.iastate.edu/courses/Geol_100/old_ files/glossary.v2.html, (ND/ Geol - Amer) {111}