The Skeptical Inquirer had a wonderful article on Pliny the Elder in a 2003 issue:
Though there may be 20,000 topics in his Natural History, the simple fact is that far too many of the “facts” Pliny provides us are not facts at all, but unverified anecdotes reported as facts. If we were to swing an imaginary “B.S.” detector over Pliny’s book, the meter would read off-scale. What do we make of this? How does it affect our judgment of poor Gaius Plinius? Is he a rampant credulist, rational skeptic, or both?
The evidence he leaves in his Natural History suggests that Pliny was no different from most of us. His belief system and the structure by which he explained the world grew naturally out of the culture in which he was raised and lived, and though he might now and then reach beyond that culture, unlike either Thales or Aristotle, Pliny was neither genius nor pioneer.
Yet Pliny stood at a significant decision point of Western history, when one pathway to the future could have followed Stoic ethics towards the close study of nature and our role in it. Instead, within a few centuries of his death the dark barbarity of the Church fell over Europe, arresting the nascent rationality of pagan philosophy. The evidence we have, as we read his Natural History, suggests Pliny was a conflicted man, with a deep belief in skepticism and rational inquiry, yet unable to rise out of the magical thinking endemic around him.
Pliny the Elder’s Natural History can be found translated in English here.