Statistical abstract of the United States  : 1991 : the national  data book  Economics and statistics Administration  Bureau of the Census 
/ United States Departement of commerce 
. - 111th ed. 
. - Washington  : U.S. : Bureau of the Census, 1991
. - XXIV-986 p.  : ill.  ; 24 cm. 
<br/><br/><label>Universal Decimal Class. No.: </label>311.312.