Obsidian trade at San Lorenzo Tenochtitlan, Mexico : analysis of obsidian artifacts emphazises the role of trade in the rise of Olmec civilization. - Washington, D.C - Cobean, Robert H. (Science ; 174.1971, no. 4010, S. 666-671 : 3 Ill. ; 1 Kt. ; 3 Tab., 1971)