What is the source of the fortune that Judge Thatcher is keeping in trust for Huck?

The Adventures of Huckleberry Finn begins where the The Adventures of Tom Sawyer leaves off. At the end of the previous novel, Huck and Tom find a treasure of twelve thousand dollars, which they divide. Judge Thatcher takes their money and invests it in the bank at six percent interest, so that each boy earns a dollar a day on their money.

Now the way that the book winds up is this: Tom and me found the money that the robbers hid in the cave, and it made us rich. We got six thousand dollars apiece—all gold. It was an awful sight of money when it was piled up. Well, Judge Thatcher he took it and put it out at interest, and it fetched us a dollar a day apiece all the year round—more than a body could tell what to do with.