( ( ( An ( indefinite mixture ) ) ( of ( ( naturally occurring ) substances ) ) ) ( , ( mainly ( minerals . ) ) ) )
(ROOT (S (NP (NP (DT An) (JJ indefinite) (NN mixture)) (PP (IN of) (NP (ADJP (RB naturally) (VBG occurring)) (NNS substances)))) (, ,) (ADVP (RB mainly)) (VP (VBN minerals)) (. .)))
An indefinite mixture of naturally occurring substances, mainly minerals .
(ROOT (S (NP (DT A) (NN mineral)) (VP (VBZ is) (VP (VBN described) (PP (IN as) (NP (NP (DT a) (JJ natural) (NN substance)) (SBAR (WHNP (WDT that)) (S (VP (VBZ makes) (PRT (RP up)) (NP (NNS rocks))))))))) (. .)))