Background samples usually consist of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood.
(Background samples; consist; of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood; T:usually)
Background samples
consist
of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood T:usually