As I said, I didn't think it would be that hard. While my post on the Answer to last week's Search Challenge is fairly long, you really should see my notes! In the process of doing the research, I ended up with about 3X what I put into the post (including some truly wonderful things that the margin couldn't contain--such as the factoid that Charles Mentry died of an untreated insect bite... go figure).
In any case, I wanted to spend this post to reflect on what I did to answer the question, and what it tells us about doing this kind of historical research.
1. There's a lot of copying going on. I was dismayed to find key "facts" and "concepts" keep recurring in the exact same language. I only gave a hint about it in my post. Truth is, copying is rampant. And it suggests a strategy for researchers: Read the text carefully enough to recognize it if you see it again.
Copied texts almost always indicate a lack of careful scholarship, and while you can sometimes figure out where the original text came from, it's also sometimes so tangled that you really can't tell. Every time I see the same text claiming the same point, I lose a little confidence.
This was especially true when I first saw the claim that oil was discovered in Humboldt County "...which is in the Central Valley..." I knew that was wrong, so whenever I saw it repeated, I knew that the repeaters hadn't bothered to look at a map to validate their copying. (Note to researchers: When you see an easily verifiable claim like that, spend the 10 seconds to check it out. Suppose the phrase had been "... the Sudd, which is near the Mau Escarpment..."--would you have any idea that the Sudd is nowhere *near* the Mau Escarpment? You could look it up...)
2. Beware of articles that don't cite anything. I found lots of history enthusiasts that make all kinds of fantastic claims... except you can't track down any of what they claim. I did find some marvelous claims, but they were written in a gush of civic pride and boosterism. If the claim sounds plausible, I might be moved to contact the author and try to run it down--but hard experience tells me that often they can't support their claims. They believe them, they truly do, but that's not the same as having the smoking gun.
3. Be happy when you start to see lots of slightly different version of the story that corroborate each other, often with minor details added from version to version. In addition to all the repetitions about the Pico No. 4 oil strike, there was plenty of variety in the stories. Some talked about Mentry and the city that grew up around Pico No. 4 (Mentryville), while others talked about the development of the oil refining industry that came out of Pico No. 4. Point is, there were LOTS of supporting stories--plenty with pictures that had dates written on them at the time.
4. Don't give up too soon--try variations on the theme. I didn't initially believe the Union Mattolle Company story because my first reference had the name of the company misspelled as "Union Mattolle," and I couldn't find ANY other references to that company. But once I tried searching for just Mattole, I was able to find lots of hits.
Finally, a note about my notetaking.
There are a gazillion ways to take notes. For clear questions like this--"what date was the first..."--it's often simplest to keep your notes organized by time.
I just opened a text-editor and copy/pasted claims into section headings by years. I had one section for 1865, another for 1879, and so on. Then I'd copy the claim (along with any notes about the claim that I'd picked up along the way, such as questions I had about its credibility, or a note that it might be copied from elsewhere) into the section by year. I'd ALWAYS include the link to the source document. (Key idea: Don't count on being able to re-find the document again. I lost at least one good reference because I couldn't refind it!)
Then, once organized like that, putting together the blog post was straight-forward--just list the top most-plausible claims and give a bit of background on each. (While bearing in mind that the final product has to be salient and interesting.) That's why the final product is so much shorter than the notes. All those wonderful side lights and tidbits belong somewhere else, not in your masterful summary.
But now we're into the realm of writing, which is another topic for another time.