Sotillo means that Monygham would be unable to actually get to, dig up, and utilize the silver on his own. But the line also suggests Nostromo's plight, and as such continues the symbolic theme in which treasure corresponds to character. The implication is that purity of character in isolation is "of no use" because one can "do nothing with it" -- that one requires a social context to perform any sort of action. It is exactly this tragic dilemma that will force Nostromo into his duplicitous role; or, allegorically speaking, force the People back into society.