About: http://webisa.webdatacommons.org/112988966

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	The depth of overburden which may be profitably removed, depends upon the thickness and purity of the deposit, but about 20 feet is the limit, except in the case of very thick beds of high grade ore.
http://www.w3.org/ns/prov#wasQuotedFrom	lenntech.com