( ( At ( ( the top ) ( of ( this area ) ) ) ) ( , ( ( patches ( of ( dark lichens ) ) ) ( ( can ( appear ( as ( crusts ( on rocks ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (NP (DT the) (NN top)) (PP (IN of) (NP (DT this) (NN area))))) (, ,) (NP (NP (NNS patches)) (PP (IN of) (NP (JJ dark) (NNS lichens)))) (VP (MD can) (VP (VB appear) (PP (IN as) (NP (NP (NNS crusts)) (PP (IN on) (NP (NNS rocks))))))) (. .)))
At the top of this area, patches of dark lichens can appear as crusts on rocks.
(ROOT (S (NP (NNP Gabbro)) (VP (VBZ is) (NP (NP (DT a) (JJ dark) (JJ dense) (NN rock)) (SBAR (WHNP (WDT that)) (S (VP (MD can) (VP (VB be) (VP (VBN found) (PP (IN in) (NP (DT the) (JJ oceanic) (NN crust)))))))))) (. .)))
Gabbro is a dark dense rock that can be found in the oceanic crust.