Check Barb Schmidt's Twainquotes. That's the date of writing; the letter appeared in the Enterprise of January 7.

    On Wednesday, May 13, 2020, 06:26:21 AM PDT, Clay Shannon <[log in to unmask]> wrote:  
 
 I have notes that Twain wrote the following:
They speak of “prodigious veins” and “magnificent deposits” and “wonderful richness,” etc., and behold their tongues are touched with inspiration and they prophecy! They reveal the things that shall come to pass, with the easy confidence of Elisha’s newly invested with the enchanted mantle.
...in an article named "J. ROSS BROWNE'S REPORT" dated December 16, 1867.
But now I can no longer locate the source of it - where did it appear? Is anybody familiar with this one?

- B. Clay Shannon