^Lavie, Henrique J. (1982).
"An Overview of Problems and Requirements - The Outlook from Venezuela". Resources for the Twenty-first Century: Proceedings of the International Centennial Symposium of the United States Geological Survey, Held at Reston, Virginia, October 14-19, 1979, Issue 1193. U.S. Geological Survey. p. 49.
^Lavie, Henrique J. (1982).
"An Overview of Problems and Requirements - The Outlook from Venezuela". Resources for the Twenty-first Century: Proceedings of the International Centennial Symposium of the United States Geological Survey, Held at Reston, Virginia, October 14-19, 1979, Issue 1193. U.S. Geological Survey. p. 49.